Early nymphal mortality in the cotton fleahopper,Pseudatomoscelis seriatus(Reuter), was increased in constant temperatures above 29.4°C or below 23.9°C. Nymphal survival in 40.6, 12.8, and 15.0°C regimens was low. No significant differences were found in length of preoviposition or oviposition periods or numbers of eggs deposited at different constant temperatures. However, fewer females deposited eggs in temperatures above 29.4°C or below 23.9°C.There was close agreement between theoretical thermal constants and observed thermal constants calculated for insects reared in constant, fluctuating, and insectary conditions where temperatures were not extreme. Predictions of developmental rates at high or low temperatures were less accurate. During the time most fleahoppers are present in the field, the thermal constant of the total life cycle (egg to egg) for fleahoppers reared on hosts as suitable as beans and potatoes is probably between 570 and 600°C-days above a developmental threshold of 7.0°C.