Comparing eight species of frogs adapted to habitats ranging from aquatic to arboreal, we observed differences in the affinity of primary olfactory projections to the lectin 'soybean agglutinin'. Particularly pronounced differences exist between Pipa, a frog that rarely leaves the water, and Eleutherodactylus, an arboreal species that does not even utilize aquatic environments for reproduction. We interpret these differences as reflecting specializations to the perception of air-borne vs. water-borne odors.