Aggregation behavior was elicited in both sexes of the green June beetle,Cotinis nitida(L.), when green June beetle males fed on peach slices inoculated with one yeast species,Trichosporon cutaneum(de Beurm. Gougerot et Voucher) Ota. Peach puree monocultures were made by inoculating sterile peach puree with one of three yeasts (Aureobasidium pullulans(de Bary) Arnaud,Candida krusei(Castellani) Berkhout,C. tropicalis(Castellani) Berkhout), which were all endemic flora isolated from the feces and gut of green June beetles collected from feeding aggregations on unsterile peaches in the field (natural bait). In a field bioassay, peach puree monocultures ofC. tropicalis(in experiment A) andC. krusei(in experiment B), both without beetle feeding, were significantly more attractive (P<0.05) to both sexes of green June beetles than sterile peach puree with or without beetle feeding, but all peach purees were significantly less attractive than the most attractive natural bait. In experiment C, peach slices were inoculated with either of two yeasts (biovars A or B ofT. cutaneum), which were the predominant isolates from the gut of laboratory-reared green June beetles fed on unsterile peaches. These monocultures ofT. cutaneumon peach slices without beetle feeding were relatively as attractive as monoculture-inoculated peach puree alone containing one of the three other yeasts noted above. But when laboratory-reared green June beetle males previously fed on theT. cutaneum-inoculated peach slices were included with these peach slices, the bait containing biovar A was as attractive as the natural bait. Although the bait of laboratory-reared green June beetle males feeding on peach slices inoculated with biovar B was significantly less attractive (P<0.05) than the natural bait, it was Similarly attractive to the laboratory-reared green June beetle males feeding on the biovar A-inoculated peach slices. The laboratory-reared green June beetles required a period of 1 d or more of feeding on yeast-inoculated fruit to achieve peak production of aggregation semiochemical.