The segmental levels of the innervation of the flexor hallucis longus (FHL) in the cat have been unclear. To clarify the location of the spinal innervation of the FHL, physiologic techniques were used both to identify the ventral roots supplying the FHL and to relate the distribution of its segmental innervation to that of another calf muscle, the soleus, in the same cat. The soleus was used as a reference because its innervation has been consistently located by several investigators using a variety of techniques. The contributions of the various lumbosacral ventral roots were assessed by measuring the muscle contractions produced by electrically stimulating individual roots. The FHL was found to be supplied only by roots L6 and L7 and the soleus, as reported by previous investigators, only by L7 and S1. In every cat, at least some part of the innervation to the FHL was rostral to that of the soleus. Comparison of these results with those of Sherrington, Romanes, and Jefferson strongly suggests that the FHL in the present study corresponds to the adjacent synergistic muscle labeled flexor digitorum longus by Sherrington and Romanes.