Four selected leg muscles (gastrocnemius, soleus, vastus lateralis and intermedius) from thirty-two humans were autopsied within 25 hr of death and examined histochemically. The results of histochemical myofibrillar adenosine triphosphatase activity demonstrated that the soleus and vastus intermedius muscles have a higher proportion of slow twitch fibres (70%, 47%) than their synergists, gastrocnemius and vastus lateralis, respectively. The gastrocnemius contains about 50% slow twitch fibres and the vastus lateralis about 32%. Similar proportions of slow and fast twitch fibres have been reported for these hindlimb muscles in other mammals. Human muscles, however, differ from other mammalian muscles in that the proportion of slow and fast twitch fibres were similar in the superficial and deep regions of the muscles examined. Fast twitch oxidative glycolytic fibres in sedentary humans were observed less frequently, and they are less prominent in terms ofoxidative enzymatic activity when compared to similar fibres of several laboratory mammals studied previously.