Abaxial pubescence of the trifoliolate leaves of numerous species is asymmetric, with a greater density of trichomes on the portions of the lateral leaflets between their midribs and outer margins (the margins closest to the base of the petiole) than on the remainder of the lateral leaflets or on the central leaflet. This pattern was evaluated statistically in 1250 plants representing 55 species of Medicago, 13 other genera of the Leguminosae, and 5 genera of other plant families. It was found that the magnitude of asymmetry of pubescence is higher in Medicago than in other legumes considered collectively, but the phenomenon is widespread in the Leguminosae. The generality of the pattern in other vascular plant families could not be assessed reliably because of the paucity of nonlegume species with pubescent trifoliolate leaves. However, the pattern is not universal because in Staphylea trifolia (Staphyleaceae) an opposite asymmetry was found. Young trifoliolate leaves of leguminous plants frequently are folded in a complex fashion, which exposes only one-sixth of the surface area, corresponding to the relatively pubescent portions. Higher trichome density on these exposed surfaces may reflect adaptation, perhaps against phytophagous insects or desiccation. However, the pattern may also be explained by a tendency for better trichome development on exposed epidermal surfaces in comparison with surfaces adpressed against each other. A second asymmetry of trifoliolate leaves was also examined: a trend for the width of the relatively pubescent portion of the lateral leaflet to be greater than that of the corresponding portion on the opposite side of the midrib. While this asymmetry was found to be significant, and to be especially well developed in the nonlegumes examined, there are numerous exceptions, and at least in the Leguminosae the tendency is not as strikingly developed as the eccentricity of pubescence.