A California strain (37°N) and a Washington strain (47°N) of Anopheles freeborni Aitken were compared for their responses to photoperiod. A sharp 1-hr difference (critical photoperiod) in response of adult female fat body was demonstrated. This critical photoperiod was 9–10 hr for the California strain, 11–12 hr for the Washington strain. These latitudinal differences in response seem explainable on the basis of expected seasonal temperatures. Ovary length was also noted to respond to photoperiod, but no consistent relationship was seen in willingness of females to take a blood meal. Both larvae and adults respond to day length.