The spatial distribution of muscle fibers belonging to a motor unit was studied in the soleus and tibialis anterior muscles of adult cats. Motor unit fibers were depleted of their glycogen through repetitive stimulation of the motoneuron or of the functionally isolated motor axon. Subsequently, the position of depleted muscle fibers was mapped on serial cross sections taken along the length of the muscle. A subset of fibers was selected from the cross section containing the largest number of motor unit fibers and 4 spatial analyses were performed. These analyses were designed to determine whether the muscle fibers belonging to a single unit were distributed in a random manner. To test whether the actual distribution was other than random, Monte Carlo techniques were used to simulate the random innervation of a muscle. From these simulations, a test statistic was calculated for comparison with the observed data. Adjacency and nearest-neighbor analyses revealed no tendency for grouping or dispersion of fibers belonging to a motor unit. However, measurement of the distances between all motor unit fibers revealed a greater tendency for grouping than spreading, suggesting the existence of some mechanism that restricts the absolute distribution and territory of a motor unit.