Population fluctuations of various species ofDrosophila in the 5 localities of Bababudangiri and Kemmangundi Hill ranges (Western Ghats) were analysed with reference to the seasons of the year. The radical changes in the environmental factors caused by the alterations of seasons were found to influence greatly the size ofDrosophila populations. The quantitative differences in the monthly collections ofDrosophila species such asD. giriensis, D. takahashii, D. mysorensis, D. nasuta andD. immigrans were found to be closely related to the climatic variations. The differential magnitude of the population peaks and their asynchronous formation by different species as observed in the present study were found to be a reflection of differential responses of these species to the ambient and changing ecological factors. The cyclical changes in the size ofDrosophila populations under study appears to be regulated by the seasonal rainfall and the related changes in the environment (extrinsic factors) coupled with genetic and physiological factors (intrinsic factors) generated within the populations.