The properties of 97 motor units in the soleus muscle of the cat were investigated by isolating and stimulating filaments of ventral roots containing only 1 soleus axon. The conduction velocities of the axons innervating these units ranged from 50.7-81.3 m/sec. The maximal tetanic tensions produced by single units ranged from 3.2-40.4 g, with a mean of 14.8 g. The distribution of unit tensions resembled a regression curve, with about 1/3 of all units falling into the 5-10 g group and decreasing numbers of units in each 5 g group from 10-40 g. The contraction-times ranged from 58-193 msec, with a peak between 80-90 msec. The per cent of maximal tension developed at rates of 5, 10, 20 and 50/sec. was compiled for all units and represented graphically. A relation between the maximum tetanic tension of a unit and the conduction velocity (or diameter) of its axon was apparent: slowly conducting fibers supplied small motor units and rapidly conducting fibers large units.