diff --git a/sgkit_plink/pysnptools.py b/sgkit_plink/pysnptools.py
index e59bdd4..2d21147 100644
--- a/sgkit_plink/pysnptools.py
+++ b/sgkit_plink/pysnptools.py
@@ -1,11 +1,12 @@
 """PLINK 1.9 reader implementation"""
 from pathlib import Path
-from typing import Optional, Union
+from typing import Optional, Union, Mapping, Any
 
 import dask.array as da
 import dask.dataframe as dd
 import numpy as np
 from dask.dataframe import DataFrame
+from dask.array import Array
 from pysnptools.snpreader import Bed
 from xarray import Dataset
 
@@ -92,11 +93,26 @@ def close(self):
         self.bed._close_bed()  # pragma: no cover
 
 
-def _to_dict(df, dtype=None):
-    return {
-        c: df[c].to_dask_array(lengths=True).astype(dtype[c] if dtype else df[c].dtype)
-        for c in df
-    }
+def _max_str_len(arr: Array) -> Array:
+    return arr.map_blocks(
+        lambda s: np.char.str_len(s.astype(str)), dtype=np.int8
+    ).max()
+
+
+def _to_dict(df: DataFrame, dtype: Mapping[str, Any]=None):
+    arrs = {}
+    for c in df:
+        a = df[c].to_dask_array(lengths=True)
+        dt = df[c].dtype
+        if dtype:
+            dt = dtype[c]
+        kind = np.dtype(dt).kind
+        if kind in ['U', 'S']:
+            # Compute fixed-length string dtype for array
+            max_len = _max_str_len(a).compute()
+            dt = f"{kind}{max_len}"
+        arrs[c] = a.astype(dt)
+    return arrs
 
 
 def read_fam(path: PathType, sep: str = " ") -> DataFrame:
diff --git a/sgkit_plink/tests/data/plink_sim_10s_100v_10pmiss.bim b/sgkit_plink/tests/data/plink_sim_10s_100v_10pmiss.bim
index 01325a9..3e668d5 100644
--- a/sgkit_plink/tests/data/plink_sim_10s_100v_10pmiss.bim
+++ b/sgkit_plink/tests/data/plink_sim_10s_100v_10pmiss.bim
@@ -1,100 +1,100 @@
-1	1:1:A:C	0.0	1	C	A
-1	1:2:A:C	0.0	2	C	A
-1	1:3:A:C	0.0	3	C	A
-1	1:4:A:C	0.0	4	C	A
-1	1:5:A:C	0.0	5	C	A
-1	1:6:A:C	0.0	6	C	A
-1	1:7:A:C	0.0	7	C	A
-1	1:8:A:C	0.0	8	C	A
-1	1:9:A:C	0.0	9	C	A
+1	1:1:G:CGCGCG	0.0	1	CGCGCG	G
+1	1:2:ACT:G	0.0	2	G	ACT
+1	1:3:ACT:G	0.0	3	G	ACT
+1	1:4:G:CGCGCG	0.0	4	CGCGCG	G
+1	1:5:G:CGCGCG	0.0	5	CGCGCG	G
+1	1:6:ACT:G	0.0	6	G	ACT
+1	1:7:G:CGCGCG	0.0	7	CGCGCG	G
+1	1:8:T:GTGG	0.0	8	GTGG	T
+1	1:9:T:GTGG	0.0	9	GTGG	T
 1	1:10:A:C	0.0	10	C	A
-1	1:11:A:C	0.0	11	C	A
-1	1:12:A:C	0.0	12	C	A
-1	1:13:A:C	0.0	13	C	A
-1	1:14:A:C	0.0	14	C	A
-1	1:15:A:C	0.0	15	C	A
+1	1:11:ACT:G	0.0	11	G	ACT
+1	1:12:G:CGCGCG	0.0	12	CGCGCG	G
+1	1:13:G:CGCGCG	0.0	13	CGCGCG	G
+1	1:14:T:GTGG	0.0	14	GTGG	T
+1	1:15:ACT:G	0.0	15	G	ACT
 1	1:16:A:C	0.0	16	C	A
-1	1:17:A:C	0.0	17	C	A
-1	1:18:A:C	0.0	18	C	A
+1	1:17:ACT:G	0.0	17	G	ACT
+1	1:18:T:GTGG	0.0	18	GTGG	T
 1	1:19:A:C	0.0	19	C	A
 1	1:20:A:C	0.0	20	C	A
-1	1:21:A:C	0.0	21	C	A
-1	1:22:A:C	0.0	22	C	A
-1	1:23:A:C	0.0	23	C	A
+1	1:21:T:GTGG	0.0	21	GTGG	T
+1	1:22:G:CGCGCG	0.0	22	CGCGCG	G
+1	1:23:T:GTGG	0.0	23	GTGG	T
 1	1:24:A:C	0.0	24	C	A
 1	1:25:A:C	0.0	25	C	A
-1	1:26:A:C	0.0	26	C	A
-1	1:27:A:C	0.0	27	C	A
-1	1:28:A:C	0.0	28	C	A
-1	1:29:A:C	0.0	29	C	A
+1	1:26:ACT:G	0.0	26	G	ACT
+1	1:27:G:CGCGCG	0.0	27	CGCGCG	G
+1	1:28:ACT:G	0.0	28	G	ACT
+1	1:29:T:GTGG	0.0	29	GTGG	T
 1	1:30:A:C	0.0	30	C	A
-1	1:31:A:C	0.0	31	C	A
-1	1:32:A:C	0.0	32	C	A
-1	1:33:A:C	0.0	33	C	A
-1	1:34:A:C	0.0	34	C	A
+1	1:31:T:GTGG	0.0	31	GTGG	T
+1	1:32:G:CGCGCG	0.0	32	CGCGCG	G
+1	1:33:ACT:G	0.0	33	G	ACT
+1	1:34:G:CGCGCG	0.0	34	CGCGCG	G
 1	1:35:A:C	0.0	35	C	A
-1	1:36:A:C	0.0	36	C	A
-1	1:37:A:C	0.0	37	C	A
+1	1:36:G:CGCGCG	0.0	36	CGCGCG	G
+1	1:37:T:GTGG	0.0	37	GTGG	T
 1	1:38:A:C	0.0	38	C	A
 1	1:39:A:C	0.0	39	C	A
-1	1:40:A:C	0.0	40	C	A
+1	1:40:T:GTGG	0.0	40	GTGG	T
 1	1:41:A:C	0.0	41	C	A
-1	1:42:A:C	0.0	42	C	A
-1	1:43:A:C	0.0	43	C	A
-1	1:44:A:C	0.0	44	C	A
-1	1:45:A:C	0.0	45	C	A
-1	1:46:A:C	0.0	46	C	A
-1	1:47:A:C	0.0	47	C	A
+1	1:42:G:CGCGCG	0.0	42	CGCGCG	G
+1	1:43:T:GTGG	0.0	43	GTGG	T
+1	1:44:ACT:G	0.0	44	G	ACT
+1	1:45:G:CGCGCG	0.0	45	CGCGCG	G
+1	1:46:ACT:G	0.0	46	G	ACT
+1	1:47:G:CGCGCG	0.0	47	CGCGCG	G
 1	1:48:A:C	0.0	48	C	A
 1	1:49:A:C	0.0	49	C	A
 1	1:50:A:C	0.0	50	C	A
-1	1:51:A:C	0.0	51	C	A
+1	1:51:G:CGCGCG	0.0	51	CGCGCG	G
 1	1:52:A:C	0.0	52	C	A
-1	1:53:A:C	0.0	53	C	A
+1	1:53:ACT:G	0.0	53	G	ACT
 1	1:54:A:C	0.0	54	C	A
-1	1:55:A:C	0.0	55	C	A
-1	1:56:A:C	0.0	56	C	A
-1	1:57:A:C	0.0	57	C	A
+1	1:55:G:CGCGCG	0.0	55	CGCGCG	G
+1	1:56:T:GTGG	0.0	56	GTGG	T
+1	1:57:G:CGCGCG	0.0	57	CGCGCG	G
 1	1:58:A:C	0.0	58	C	A
-1	1:59:A:C	0.0	59	C	A
-1	1:60:A:C	0.0	60	C	A
-1	1:61:A:C	0.0	61	C	A
+1	1:59:T:GTGG	0.0	59	GTGG	T
+1	1:60:G:CGCGCG	0.0	60	CGCGCG	G
+1	1:61:ACT:G	0.0	61	G	ACT
 1	1:62:A:C	0.0	62	C	A
-1	1:63:A:C	0.0	63	C	A
-1	1:64:A:C	0.0	64	C	A
-1	1:65:A:C	0.0	65	C	A
-1	1:66:A:C	0.0	66	C	A
-1	1:67:A:C	0.0	67	C	A
-1	1:68:A:C	0.0	68	C	A
-1	1:69:A:C	0.0	69	C	A
-1	1:70:A:C	0.0	70	C	A
-1	1:71:A:C	0.0	71	C	A
-1	1:72:A:C	0.0	72	C	A
+1	1:63:G:CGCGCG	0.0	63	CGCGCG	G
+1	1:64:T:GTGG	0.0	64	GTGG	T
+1	1:65:T:GTGG	0.0	65	GTGG	T
+1	1:66:ACT:G	0.0	66	G	ACT
+1	1:67:T:GTGG	0.0	67	GTGG	T
+1	1:68:ACT:G	0.0	68	G	ACT
+1	1:69:G:CGCGCG	0.0	69	CGCGCG	G
+1	1:70:G:CGCGCG	0.0	70	CGCGCG	G
+1	1:71:ACT:G	0.0	71	G	ACT
+1	1:72:G:CGCGCG	0.0	72	CGCGCG	G
 1	1:73:A:C	0.0	73	C	A
 1	1:74:A:C	0.0	74	C	A
-1	1:75:A:C	0.0	75	C	A
+1	1:75:T:GTGG	0.0	75	GTGG	T
 1	1:76:A:C	0.0	76	C	A
-1	1:77:A:C	0.0	77	C	A
-1	1:78:A:C	0.0	78	C	A
+1	1:77:ACT:G	0.0	77	G	ACT
+1	1:78:ACT:G	0.0	78	G	ACT
 1	1:79:A:C	0.0	79	C	A
 1	1:80:A:C	0.0	80	C	A
 1	1:81:A:C	0.0	81	C	A
-1	1:82:A:C	0.0	82	C	A
+1	1:82:T:GTGG	0.0	82	GTGG	T
 1	1:83:A:C	0.0	83	C	A
-1	1:84:A:C	0.0	84	C	A
+1	1:84:ACT:G	0.0	84	G	ACT
 1	1:85:A:C	0.0	85	C	A
-1	1:86:A:C	0.0	86	C	A
-1	1:87:A:C	0.0	87	C	A
+1	1:86:G:CGCGCG	0.0	86	CGCGCG	G
+1	1:87:ACT:G	0.0	87	G	ACT
 1	1:88:A:C	0.0	88	C	A
 1	1:89:A:C	0.0	89	C	A
-1	1:90:A:C	0.0	90	C	A
-1	1:91:A:C	0.0	91	C	A
-1	1:92:A:C	0.0	92	C	A
+1	1:90:T:GTGG	0.0	90	GTGG	T
+1	1:91:T:GTGG	0.0	91	GTGG	T
+1	1:92:T:GTGG	0.0	92	GTGG	T
 1	1:93:A:C	0.0	93	C	A
 1	1:94:A:C	0.0	94	C	A
 1	1:95:A:C	0.0	95	C	A
 1	1:96:A:C	0.0	96	C	A
-1	1:97:A:C	0.0	97	C	A
-1	1:98:A:C	0.0	98	C	A
-1	1:99:A:C	0.0	99	C	A
+1	1:97:T:GTGG	0.0	97	GTGG	T
+1	1:98:ACT:G	0.0	98	G	ACT
+1	1:99:T:GTGG	0.0	99	GTGG	T
 1	1:100:A:C	0.0	100	C	A
diff --git a/sgkit_plink/tests/data/plink_sim_10s_100v_10pmiss.fam b/sgkit_plink/tests/data/plink_sim_10s_100v_10pmiss.fam
index e0af7f4..36efeff 100644
--- a/sgkit_plink/tests/data/plink_sim_10s_100v_10pmiss.fam
+++ b/sgkit_plink/tests/data/plink_sim_10s_100v_10pmiss.fam
@@ -1,10 +1,10 @@
-0	0	0	0	0	NA
-0	1	0	0	0	NA
-0	2	0	0	0	NA
-0	3	0	0	0	NA
-0	4	0	0	0	NA
-0	5	0	0	0	NA
-0	6	0	0	0	NA
-0	7	0	0	0	NA
-0	8	0	0	0	NA
-0	9	0	0	0	NA
+0	000	0	0	0	NA
+0	001	0	0	0	NA
+0	002	0	0	0	NA
+0	003	0	0	0	NA
+0	004	0	0	0	NA
+0	005	0	0	0	NA
+0	006	0	0	0	NA
+0	007	0	0	0	NA
+0	008	0	0	0	NA
+0	009	0	0	0	NA
diff --git a/sgkit_plink/tests/test_pysnptools.py b/sgkit_plink/tests/test_pysnptools.py
index ac5df4d..5d38578 100644
--- a/sgkit_plink/tests/test_pysnptools.py
+++ b/sgkit_plink/tests/test_pysnptools.py
@@ -32,6 +32,17 @@ def test_raise_on_both_path_types():
         read_plink(path="x", bed_path="x")
 
 
+def test_fixlen_str_variable(ds1):
+    assert ds1["sample_id"].dtype == np.dtype("<U3")
+    assert ds1["variant_id"].dtype == np.dtype("<U13")
+    assert ds1["variant_allele"].dtype == np.dtype("|S6")
+    assert ds1["sample_family_id"].dtype == np.dtype("<U1")
+    # TODO: Remove 'None' strings https://github.com/pystatgen/sgkit-plink/issues/16
+    # which should make these <U1
+    assert ds1["sample_maternal_id"].dtype == np.dtype("<U4")
+    assert ds1["sample_paternal_id"].dtype == np.dtype("<U4")
+
+
 def test_read_slicing(ds1):
     gt = ds1["call_genotype"]
     shape = gt.shape