1. A new preparation of the thoracic nervous system of the crayfish, Procambarus clarkii, has been developed, in which it is possible to work with identified members of motor neuronal pools. 2. In such a preparation, it is possible to dissect all specific proximal motor nerves (protractor, retractor, anterior elevator, posterior elevator, and depressor). Motor neurons innervating the four proximal muscles of the fourth walking leg have been identified both physiologically and anatomically by staining the recorded motor neuron with Lucifer yellow through the microelectrode. 3. By the use of cobalt chloride, we have mapped the distribution of somata of all motor neurons within the fourth thoracic ganglion that innervate the different groups of muscles controlling the movement of the fourth walking leg. 4. Most motor neurons innervating the same muscle seem to be electrically coupled, except some depressor motor neurons. 5. Motor neurons innervating antagonist muscles are linked by inhibitory connections. These connections are reciprocal for protractor and retractor motor neurons but usually not reciprocal between elevator and depressor motor neurons. 6. Walking interneurons were identified as neurons without axons in any motor nerve, which modified the motor neuronal activity. Some of them have been injected with Lucifer yellow. 7. Some interneurons make synaptic connections only with antagonist motor neurons that control the movement of one joint. Probably their functional role is to reinforce or to limit the antagonism between each pair of antagonist motor neurons. 8. Other interneurons make synaptic connections with motor neurons innervating muscles controlling different leg joints. These interneurons may play a role in generating the motor patterns that underlie forward and backward walking.