1. Isolated, unstimulated abdominal ventral nerve cords ofLimulus display patterns of motor output characteristic of rhythmic gill ventilation and of gill cleaning (Fig. 2). 2. The motor output may occur as long-term patterns of alternate gill cleaning and ventilation (Fig. 3), or alternate gill cleaning and tonic activity (Fig. 4). 3. Two patterns of gill cleaning motor output, apparently corresponding to the left-leading and right-leading patterns observed in intact animals, persist in the isolated ventral cord (Fig. 4). These two patterns occur with the same rough alternation as in intact animals. 4. Thus all the patterns of gill-plate movement in intact animals, except those involving swimming, have underlying motor programs that are expressed in isolated nerve cords in the absence of stimulation or of sensory feedback. These findings extend the concept of central pattern generators to include complex and long-duration patterns of stereotyped behavior such as those underlying the movements of theLimulus abdominal appendages.