The increasing concern about vector-borne diseases such as West Nile disease in northern Italy motivated our analysis of data on the mosquito fauna and the seasonal and daily flight patterns collected in 1998 in the Po Valley. Collections were performed once a week from May to November, with human landing collections and CO₂ traps. Culex pipiens was present from July to October and showed a clearly unimodal nocturnal flight habit. Culex modestus appeared in July–August and showed a bimodal flight pattern, (main peak during the evening and a minor one in the morning). Aedes caspius was present from May to November (highest densities in July–August) and showed a bimodal flight pattern with a major crepuscular peak and a minor dawn peak in the morning. Aedes detritus was the most abundant species in May, with a crepuscular sharply bimodal flight pattern, particularly according to human landing collections. Sunset and sunrise time, in combination with the solar phase (that determines daylight duration and its trend of changing) were the main factors affecting flight behavior. Temperature, relative humidity, and wind speed differently affected the flight behavior of mosquito females according to the species.