Defecation in the nematode Caenorhabditis elegans is achieved by a cyclical stereotyped motor program. The first step in each cycle is contraction of a set of posterior body muscles (pBoc), followed by contraction of a set of anterior body muscles (aBoc), and finally contraction of specialized anal muscles that open the anus and expel intestinal contents (Exp). By testing existing behavioral mutants and screening for new mutants that become constipated due to defects in defecation, I have identified 18 genes that are involved in defecation. Mutations in 16 of these genes affect specific parts of the motor program: mutations in two genes specifically affect the pBoc step; mutations in four genes affect the aBoc step; mutations in four genes affect the Exp step; and mutations in six genes affect both aBoc and Exp. Mutations in two other genes affect the defecation cycle period but have a normal motor program. Sensory inputs that regulate the cycle timing in the wild type are also described. On the basis of the phenotypes of the defecation mutants and of double mutants, I suggest a formal genetic pathway for the control of the defecation motor program.