In most insects, differences between the sexes involve not only primary characters necessary for gamete production, copulation, and oviposition, but also sexual structures and behaviors involved in the acquisition of mates. The most striking of these secondary characters, such as exaggerated size, weaponry, or coloration, tend to occur in males, and have been claimed to be the full or partial result of sexual selection, reflecting competition among males for females, and female choice (Darwin, 1871; Fisher, 1930; Mayr, 1972). Documented examples of the potential for sexual selection in insects are provided by the experimental studies of mate selection in the Drosophilidae (Bateman, 1948; Petit and Ehrman, 1969; Spiess and Spiess, 1969; Ehrman, 1972), in which it is demonstrated that some male individuals are more often chosen than others. However, in most cases the precise nature of the variation upon which female choice operates (e.g., "male vigor") has been difficult to characterize (Alexander, 1975; Thornhill, 1979). Male-male competition for females has also been described, and species-specific competitive techniques compared (Richards, 1927; Moore, 1952; Johnson, 1962; Parker, 1970, 1974; Ziegler, 1972; Campanella and Wolf, 1974; Eberhard, 1979; Hamilton, 1979; McAlpine, 1979; Waage, 1979). However, there have been relatively few attempts to assess the consequences of individual variation for differential reproductive success in insects, especially in natural populations (Thornhill, 1976a, 1976b, 1976c, 1979; Scheiring, 1977; McCauley and Wade, 1978; McCauley, 1979; Mason, 1964, 1980). I report here the results of a study of the influence of body size on mate preference and on success in intrasexual competition, in a natural aggregation of the neotropical brentid weevil, Brentus anchorago L. (Coleoptera, Curculionoidea, Brentidae). Brentids make good candidates for studies of sexual selection and individual variation because most species of the family exhibit pronounced sexual dimorphism (Muizon, 1960; Haedo Rossi, 1961; Damoiseau, 1967, 1971). The males generally possess greater body length, a stouter rostrum, and more powerful mandibles, one of which may be grossly enlarged (Darwin, 1871). Within each sex there is impressive phenotypic variation in body size, especially in males, which fight one another with snout and mandible for access to females (Wallace, 1869; Meads, 1976). The most size-variable brentid may be B. anchorago: after examining a large series of this species, Sharp (1895) commented that "the variation in length is enormous, and perhaps not equalled in the case of any other species of Coleoptera, small males being only 10-1I mm long, while large examples of the same sex attain 52 mm." Such variation in size is common within a single aggregation, and is important in male mating success, in female choice, and in patterns of mating in the aggregation as a whole.