Drosophila mauritiana larvae are tolerant to ethanol as a supplementary nutritional resource at low concentrations. Higher levels of ethanol (5 per cent) in the food adversely affect larval behaviour causing reduced rates of locomotion and feeding which are associated with low larva-to-adult survival. Two codominant autosomal alleles AdhS and AdhF controlling variant electromorphs of alcohol dehydrogenase are indistinguishable in their effects on ethanol tolerance in this species. A comparison of larval behaviours shows that D. mauritiana has a lower ethanol tolerance than either of its sibling species D. melanogaster and D. simulans.