Developmental periods of 5 strains of Anthonomus grandis Boheman from cultivated cotton ranged from 88 days at 15 to 17 days at 30; a temperature of 35 increased the period to 17.5 days. Developmental periods of I strain of thurberia weevils, A. grandis thurberiae Pierce, ranged from 72.5 days at 15°C to 17.5 days at 30; a temperature of 35°C extended the developmental period of the weevils to 18.5 days. Temperatures above an undetermined temperature between 30° and 35°C prolonged the developmental period of all strains. The deleterious effect of high temperatures partially explains the absence of boll weevil populations in Arizona in midsummer. Rearing at fluctuating temperatures did not increase the time required for development over that required at the constant temperatures if the heat input was similar and temperatures did not exceed 30°C.