diversity of Drosophila habitats attests to the evolutionary success of species in the genus. Drosophilae have been bred from a variety of substrates which include decaying plant tissue (fruits, stems, bark, wood, leaves and flowers), slime fluxes and exudates of trees and mushrooms (Carson, 197 1). Most of these habitats share the common characteristic of an abundant yeast microflora. yeasts are saprophytic and serve to process the raw materials of the habitat into important dietary factors of the larval and adult stages of the Drosophila which feed on the decomposing substrate. In this paper, the yeast communities of various Drosophila habitats are compared. Such a comparison should tell us if evolution and diversification by the yeasts has any relevance to the evolution and speciation in the drosophilae. rationale is as follows. First, it is postulated that the physiological abilities of a yeast community partially characterize the habitat of that community. Traditionally yeast taxa are described and identified by studying purified strains for a number of metabolic and physiological attributes as well as other traits (van der Walt, 1970). Thus, the information available from the taxonomy can be used directly to characterize the yeast community of a particular habitat. Second, if the yeast community characteristics are indicators of habitat characteristics, then a comparison of different yeast communities is a comparison of different habitats which contain them. A comparison of a wide variety of yeast communities could then be used to construct a classification (e.g., phenogram) of the habitats according to the characteristics of their respective yeast communities. This classification of the habitats would then relate habitats in accordance with their nutritional state of utilization by the drosophilae. Third, the classification of the habitats could then be utilized to follow and study the evolutionary pathways of groups of Drosophila for which phylogenetic arguments have been made. As H. L. Carson (1971) states: The time now appears ripe to exploit the knowledge of breeding sites for an understanding of the genetic events which occur in natural populations and which are responsible for changes leading to adaptation and speciation.