We studied the fiber type composition and contractile properties of mouse soleus motor units at 2 days, 5 days and 2 weeks of age. We used Lucifer Yellow injection to mark muscle fibers belonging to the same motor unit in the two youngest age groups, and the traditional method of glycogen depletion in the oldest. The age groups were chosen because 2 days is at the end of muscle fiber production; 5 days is at the start of synapse elimination in the muscle and 2 weeks is at the end. Muscle fibers were classified as fast (F) or slow (S) on the basis of their myosin heavy chain (MHC) content, as determined by different monoclonal antibodies. Motor units are already dominated by either F- or S-fibers at 2 days, suggesting an early preferential innervation of the two types of fibers. A substantial part of the remaining refinement of the innervation takes place during the next 3 days, while the total number of terminals in the muscle remains constant. This is most easily explained by an exchange of aberrant for correct synapses during this period. A smaller part of the refinement of the innervation occurs during the subsequent period of synapse elimination.