Seasonal variation in toxicity for white mice of intraperitoneal injections of triturated cephalothoraces of freshly caught black widow spiders, Latrodectus mactans (Fabr., 1775), was demonstrated. Spiders used in these tests were collected at Camp Bullis, Texas. Twenty-eight day old white mice from a single source were used as test animals. Intraperitoneal injections were of varying strength with respect to fraction of a cephalothorax included in a constant volume of 0.25 ml of saline solution. LD50's were expressed as fractions of a venom gland contained in the injection. Highest toxicity, as indicated by LD50's of .33 and .23, occurred in November of 1958 and 1959. Lowest toxicity, as shown by LD50's of 3.29 and 2.45, occurred in April and May, 1959.