1. Responses to mechanical and electrical stimulation have been investigated in single motor fibres dissected in the caudal nerves of the Australian yabbie Cherax destructor. These are compared with the responses of insect motor neurones.
2. A large proportion of fibres possessed a background discharge which could be accelerated but rarely inhibited on stimulation. Pre-synaptic stimulation re-set the spontaneous rhythm of many of these units.
3. Efferent responses were dependent both on the route and intensity of afferent stimulation varying in type, delay and regularity, and resembled those of crayfish interneurones.
4. Particular emphasis on the effects of the frequency of stimulation demonstrated an enhanced responsiveness of some yabbie units dependent on repetition rate. This was found in relatively few fibres and does not appear as important as in the insect.
