Field data collected from the major citrus-producing regions of Florida over the past 5 yr indicate that the introduction of the multicolored Asian lady beetle, Harmonia axyridis (Pallas), has increased in abundance while the formerly dominant Cycloneda sanguinea (L.) has declined. What is known of the diet breadth, habitat preferences, and thermal thresholds for development of these two species indicates considerable niche overlap. The larger H. axyridis has many intrinsic advantages over C. sanguinea, including higher fecundity and fertility, and a lower rate of larval cannibalism. In laboratory tests, adults and larvae of H. axyridis consumed eggs of C. sanguinea more readily than vice versa. All H. axyridis adults provided with either a conspecific or a heterospecific second-instar larva killed and ate it, whereas only 15% of C. sanguinea adults consumed a second-instar larva of H. axyridis over a 24-h period. Larvae of H. axyridis were more aggressive against larvae of C. sanguinea in laboratory tests than vice versa; even when H. axyridis larvae were paired with a C. sanguinea larva 2 d older and two to three times as large, they won 67% of contests. In choice tests, larvae of both species preferred to feed on dead C. sanguinea larvae than on dead H. axyridis larvae. Larvae of H. axyridis were capable of completing development exclusively as intraguild predators on C. sanguinea larvae, whereas the reverse was not true. The data suggest that H. axyridis is a highly evolved interspecific competitor, whereas C. sanguinea is adapted more to intraspecific competition and has few defenses against H. axyridis. Competitive displacement of C. sanguinea by H. axyridis appears to be in progress in the citrus ecosystem in Florida.