Microflora species and volatiles emitted from artificial diets were examined from the larvae of three homozygous alcohol dehydrogenase (Adh) strains of the olive fruit fly, Bactrocera (Dacus) oleae (Gmelin), reared under identical conditions. Differences in volatile composition were detected when Adh-I homozygous larvae developed in a diet lacking the preservative p-hydroxy-benzoic acid methyl ester (nipagin). Larval development of the Adh-I strain in the preservative-free diet was reduced by 50%, whereas pupal emergence was completely inhibited. The larval development and pupal emergence of Adh-F and Adh-S strains were not affected. Unique microorganisms with characteristic volatile profiles were isolated from the preservative-free diet of the Adh-I strain that were different from those, isolated from Adh-S, Adh-F, laboratory colony, and wild insect populations. Our results indicated that the variations in volatile composition of the artificial diets, and the inhibition of larval development and pupal emergence in Adh-I strain were related to changes in the microflora that developed in the diets of the Adh-I strain.