Using a dual-choice olfactometer, the role of L-lactic acid was investigated in relation to host-seeking and selection by female Anopheles gambiae Giles sensu stricto (Diptera: Culicidae) mosquitoes in a Y-tube bioassay. L-lactic acid alone was not attractive, but it significantly augmented the attractiveness of CO2, skin odour and skin-rubbing extracts from humans and other vertebrates. Comparing the left and right index fingers of the same person, one could be made more attractive than the other by adding L-lactic acid to the air stream over that finger. The difference in L-lactic acid concentration between the two air streams offered to the mosquitoes fell within the natural range of variation emanating from a human hand, suggesting that L-lactic acid modulates intraspecific host selection by An. gambiae. Analysis of skin rubbings from various vertebrates (carnivores, chickens, primates, rodents, ungulates) indicated that humans have uniquely high levels of L-lactic acid on their skin. Tests with extracts of skin rubbings from cows and humans, with and without added L-lactic acid, suggest that naturally lower levels of L-lactic acid contribute to the lesser attractiveness of non-humans to An. gambiae s.s.