The biodegradability of 2-mercaptobenzothiazole (2- MBT) under aerobic conditions was determined by assaying concentrations of 14C-carbon dioxide in controlled systems. Quantities of 14C-carbon dioxide released over the 28-day study period as a result of aerobic microbial degradation of 14C-2- mercaptobenzothiazole (14C-2-MBT) showed 0.1 to 0.2 % degradation (the same degradation value obtained in the presence of an inhibitor, sodium azide) of test substance as compared to 78.4% conversion of a reference material (14C-glucose), indicating 2-MBT did not readily biodegrade under test conditions. Batch equilibrium tests of 14C-2-MBT using 3 soil and 3 sediment types indicate its mobility is medium to low in soil and slight to immobile in sediments. An early life-stage exposure (60 days post- hatch) of rainbow trout (Oncorhynchus mykiss) to a measured concentrations of 78 ug/L 2-MBT and higher led to decreased larval length. The estimated Maximum Acceptable Toxicant Concentration (MATC) was between 41 and 78 ug /L (geometric mean MATC = 57 ug/L). Exposure of Daphnia magna for 21 days under flow-through conditions identified a 21-day EC50 of greater than 470 ug/L and MATC (survival) between 240 and 470 ug/L (geometric mean MATC = 340 ug/L). Oral doses of up to 1800 mg/kg/day by gavage to pregnant rats on gestation days 6 through 15 are not teratogenic nor developmentally toxic, but 1200 mg/kg/day and higher induces maternal toxicity (clinical signs, reduced body weight gain and food intake). Similar tests with rabbits led to slight maternal toxicity at 300 mg/kg/day (highest dose tested), but not developmental toxicity nor teratogenicity. Dose-related decreased body weight and food consumption occurred in male rats fed diets containing 8,750 ppm 2-MBT, but no dominant lethal effect was observed.