Seasonal fluctuations in Culex and Aedes were followed from 1984 to 1987 using degree-days (degree Days). The annual population maxima of a "mosquito season" can be predicted. Culex first becomes a problem at about 1,550 degree Days and disappears at 2,675 degree Days. Aedes are present at 525 degree Days, experience a midsummer decline at 900 degree Days, then resurge at 2,400 degree Days to mostly disappear at 2,700 degree Days. This timing appears to work in the arid western valley of Grand Junction, Colorado, due to the dependence by the mosquito on breeding sites related to local irrigation systems, which are dry in the winter and rehydrated at the same time each year.