In subarctic Canada conventional methods of correlation and regression analyses were inadequate in relating periodicities in mosquito flight to fluctuating physical environments (Haufe, 1958). The periodicity for individual species shifted in relation to sidereal time. In addition to a random component in the variation, a gradual shift was associated with the advance of the season for consecutive cycles of activity. For the most abundant species it appeared as older individuals constituted an increasing proportion of the natural populations. The relative variations in absolute values of light and hygrothermal conditions identified with 24-hour flight periodicities were inconsistent with seasonal variations in 24-hour meteorological cycles. If light was a primary factor in the activation of natural populations in the 24-hour cycles, observed variations in correlations would suggest variable adaptation of mosquitoes to previous light conditions and to other physical and biological factors.