The ability of threeDrosophilaspecies;Drosophila obscura, D. subobscuraandD. phalerata,to utilize a range of naturally occurring yeasts as food sources was determined. The yeasts used were the dominant species of two commonly used breeding sites,Sorbus aucupariafruits and the fruiting bodies of the fungusPhallus impudicus.The range of yeast species used was found to reflect the degree ot substrate specialization exhibited by these three species, withD. subobscurabeing broader niched than the other two species. There was an indication thatD. phaleratawas adapted to those yeasts normally found on its natural breeding site,Phallus impudicus.Many factors were found to affect the nutritional adequacy of the yeasts, including their growth period, the larval density, the presence of competitors and particularly the chemical composition of the basal growth medium.