Previous studies demonstrated that larvae from an Indian strain (S) of the seed beetle Callosobruchus maculatus (F.) are much more competitive than larvae from a Nigerian strain (I) during both intra- and interstrain competition in a small host seed (mung bean). The competitive superiority of the S strain was Similarly evident when two larvae cooccurred in a larger host species (adzuki bean) that represented a novel host for both strains. If one or both larvae within a seed belonged to the S strain, survival to adult emergence was 50% lower than it was in the absence of competition. In contrast, survival decreased by only 15% when two I-strain larvae shared a seed. In head-to-head encounters between strains, most seeds produced a single survivor, and nearly 75% of adults from such seeds belonged to the S strain. Reciprocal crosses suggested an additive genetic component to these differences in competitive ability; the reduction in survival following competition was intermediate among F 1 progeny. In contrast, the genetic basis of strain differences in adult weight was largely nonadditive, with hybrid progeny displaying a strong dominance deviation toward the heavier S strain. Differences in competitive ability were therefore independent of differences in body size between strains. Variation in competitive ability may ultimately be a consequence of variation in the sizes of host seeds typically encountered by beetle populations.