Nematodes remain important causative agents of diseases. While several studies have examined how nematodes behave in response to a chemoattractant, how the characteristics of the chemoattractant affect chemotaxis has yet to be explored. A mathematical model was created to examine how characteristics of chemoattractants affect chemotaxis in Caenorhabditis elegans. Specifically, toxic bacteria were engineered to express a chemoattractant. Under static conditions, the chemoattractant’s diffusion rate was proportional to attraction. If the nematodes learn that the chemoattractant is associated with toxicity, attraction is counterintuitively reduced with increasing diffusion rate. Results may have implications in novel treatments of parasitic infections.