Two classes of X-linked behavioral mutants of Drosophila melanogaster, leg-shaking mutants and bang-sensitive mutants, are suppressed by nap(ts) (no action potential, temperature-sensitive), an autosomal temperature-sensitive paralytic mutation. So far, nap(ts) is found to suppress thirteen mutations at seven loci, two of which produce leg shaking and five bang-sensitivity. Suppression is recessive, occurs at temperatures permissive for nap(ts), and is indirect and function-specific rather than allele-specific. At restrictive temperatures, nap(ts) is known to completely block all nerve activity. Several of the mutants suppressed by nap(ts) are shown by neurophysiological experiments to have increased nerve excitability. The physiological defect of these mutants as well as their behavioral defect is suppressed by nap(ts). Thus, suppression occurs within individual neurons at the level of excitable membranes and apparently depends on the reduction in membrane excitability caused by nap(ts) even under permissive conditions. We suggest that all mutants suppressed by nap(ts) may have related defects leading to enhanced nerve excitability. Genetic interactions of this type help reveal functional relationships between different behavioral mutants and suggest ways of isolating new mutants with altered excitable membranes.