The developmental and reproductive biology of the blue alfalfa aphid, Acyrthosiphon kondoi Shinji, was studied under 15 temperature/photoperiod regimes. Five temperatures (10°,15°,20°,25°, 30°C) and three photophases (8, 12,16 h per 24 h) were utilized in controlled environmental chambers. Temperature played the most significant role in the development and reproduction of the aphid. Optimum temperatures for development ranged from 20° to 25°, and the upper survival temperature ranged from 25° to 30°, depending upon the photoperiod. The lethal temperature was 30°. Higher survival and total fecundity occurred at low temperatures (10° to 15°) rather than at high temperatures (20° to 25°). There were significant temperature by photoperiod interactions on the rates of nymphal development, fecundity, longevity, and generation time. The changes in these rates were most prevalent at low temperatures. Shorter generation and doubling times, and a greater intrinsic rate of increase by A. kondoi, may account for the early dominance of this species over A. pisum in the spring in the U.S. alfalfa fields.