Defining a behavior that requires the function of specific neurons in the free-living nematode Caenorhabditis elegans can allow one to screen for mutations that disrupt the specification or function of those neurons. We identified serotonin-immunoreactive neurons required for tail curling or “turning” behavior exhibited by C. elegans males during mating. Males mutant in three different genes that reduce serotonin expression, cat-1, cat-4, and bas-1, exhibited defects in turning behavior similar to those of wild-type males in which these neurons were ablated. The turning defect of cat-4 males was rescued by exogenous serotonin, consistent with the idea that their behavioral defect is caused by a lack of serotonin. While the serotonin-deficient mutants we analyzed shared certain behavioral traits, they were blocked for serotonin synthesis at different steps. Analysis of these and additional serotonin-deficient mutants may help us understand how a neuron controls the expression of a serotonergic phenotype.