1. The motor effects evoked by stimulation of each of eight command fibres in the circumoesophageal commissures of the crayfish are described.

2. The fibres obtained appeared to have widespread connexions in all or most of the lower ganglia. For certain fibres the response was stronger on the homolateral side of the animal; for others it was symmetrical.

3. The frequency of stimulation of a command fibre generally had a pronounced influence on the speed of the evoked response. In addition, segments of the total response could be elicited selectively by alteration of the frequency and duration of stimulation.

4. Although the responses associated with most of the fibres were not sensitive to the fine temporal pattern of the applied stimulation, for one fibre the motor output depended clearly on the spacing of the stimulating pulses.

Supported by grant GB-2785 from the National Science Foundation and grant NB-03627 from the National Institutes of Health, Public Health Service.