Lysine and chloride ions are water-soluble attractants for Caenorhabditis elegans. When chemotaxis behavior to either of these attractants was assayed separately, the radial concentration gradients of 3 M lysine and 0.1 M ammonium chloride had similar potencies for attracting worms. However, when the concentration gradients of lysine and ammonium chloride at these concentrations were presented simultaneously, worms preferred lysine to ammonium chloride more than expected from the results obtained in separate experiments, suggesting the presence of an interaction between these two sensory information pathways within the nervous system. Chemotaxis behavior toward the radial concentration gradient of one of these attractants superimposed on a uniform concentration of the other attractant showed that the chemotaxis was augmented or attenuated by the ammonium chloride background depending on the background concentration, and attenuated by the lysine background, further supporting the interaction between the two sensory information pathways.