Temperature-dependent simulation models for nondiapause and postdiapause development of alfalfa blotch leafminer pupae,Agromyza frontella(Rondani), were used to analyze the late summer and fall development of pupae under field and laboratory conditions. Pupal development during the second summer generation was accurately simulated by the nondiapause model under both laboratory and field temperature conditions. Adult emergence in the field during the fall generation was overestimated by the nondiapause model used in conjunction with recorded field soil temperatures. This suggested that diapause occurred in the fall generation, although prolonged pupal development was not readily observed in the laboratory. The development of fall generation pupae collected from the field in October and November and reared under controlled temperatures was adequately simulated by the postdiapause model but not the nondiapause model. The results suggest that a fall generation pupal diapause occurs and is largely completed by November.