Acute bioassys were conducted on the planarian, Dugesia tigrina, to determine the toxicity of zinc, nickel, cadmium, copper, chromium, diquat, and ABS. The order of toxicity from most toxic to least toxic was: copper cadmium nickel zinc ABS chromium diquat. From these LC₅₀ values sublethal concentrations were arbitrarily chosen for use in the light response studies. After acclimation to total darkness, crawling speed away from a beam of light was measured in the controls and the experimentals at 24, 43, and 96 hours. There were significant differences between the lowest sublethal concentrations and the highest sublethal concentration used in these experiments. For zinc, cadmium, and chromium the crawling speeds were slowest in the lowest concentration used. Under nickel, copper, diquat, and ABS stress, the slowest crawling speeds were found in the highest sublethal concentration used. This suggests that different systems in the animal's body were affected by the toxicants. It appears that sublethal concentrations of these toxicants have a marked effect on the negative phototactic response of D. tigrina as measured by the crawling speed away from a beam of light. The exact mechanism for reduced crawling speeds is unknown. Speculation on the fate of the toxicant as it relates to the animal is offered. / Doctor of Philosophy