diff --git a/avogadro/qtgui/rwmolecule.cpp b/avogadro/qtgui/rwmolecule.cpp
index e4ef088cc..a97f23879 100644
--- a/avogadro/qtgui/rwmolecule.cpp
+++ b/avogadro/qtgui/rwmolecule.cpp
@@ -22,6 +22,7 @@
 #include <cassert>
 
 #include <avogadro/core/avospglib.h>
+#include <avogadro/core/spacegroups.h>
 
 namespace Avogadro {
 namespace QtGui {
@@ -1229,6 +1230,25 @@ bool RWMolecule::symmetrizeCell(double cartTol)
   return true;
 }
 
+bool RWMolecule::fillUnitCell(unsigned short hallNumber, double cartTol)
+{
+  // If there is no unit cell, there is nothing to do
+  if (!m_molecule.unitCell())
+    return false;
+
+  // Make a copy of the molecule to edit so we can store the old one
+  // The atom positions and numbers of atoms may change
+  Molecule newMolecule = m_molecule;
+
+  Core::SpaceGroups::fillUnitCell(newMolecule, hallNumber, cartTol);
+
+  Molecule::MoleculeChanges changes = Molecule::Added | Molecule::Atoms;
+  QString undoText = tr("Fill Unit Cell");
+
+  modifyMolecule(newMolecule, changes, undoText);
+  return true;
+}
+
 void RWMolecule::emitChanged(unsigned int change)
 {
   m_molecule.emitChanged(change);
diff --git a/avogadro/qtgui/rwmolecule.h b/avogadro/qtgui/rwmolecule.h
index 2d0996a82..178c673b2 100644
--- a/avogadro/qtgui/rwmolecule.h
+++ b/avogadro/qtgui/rwmolecule.h
@@ -487,6 +487,14 @@ class AVOGADROQTGUI_EXPORT RWMolecule : public QObject
    */
   bool symmetrizeCell(double cartTol = 1e-5);
 
+  /**
+   * Fill unit cell using transforms for the space group. Changes are emitted.
+   * @param hallNumber The hall number to be used for transforming the cell.
+   * @param cartTol Cartesian tolerance for symmetrization.
+   * @return True if the algorithm succeeded, and false if it failed.
+   */
+  bool fillUnitCell(unsigned short hallNumber, double cartTol = 1e-5);
+
   /**
    * @brief Begin or end an interactive edit.
    *
diff --git a/avogadro/qtplugins/spacegroup/spacegroup.cpp b/avogadro/qtplugins/spacegroup/spacegroup.cpp
index 429631723..24c7b3d43 100644
--- a/avogadro/qtplugins/spacegroup/spacegroup.cpp
+++ b/avogadro/qtplugins/spacegroup/spacegroup.cpp
@@ -319,8 +319,7 @@ void SpaceGroup::fillUnitCell()
 
   unsigned short hallNumber = view->currentIndex().row() + 1;
 
-  Core::SpaceGroups::fillUnitCell(*m_molecule, hallNumber, m_spgTol);
-  m_molecule->emitChanged(Molecule::Added | Molecule::Atoms);
+  m_molecule->undoMolecule()->fillUnitCell(hallNumber, m_spgTol);
 }
 
 void SpaceGroup::setTolerance()