The soldier beetle, Chauliognathus distinguendus, which defends inflorescences of the tree Coccoloba floribunda against intrusion by conspecific individuals, is unusual among insects in that intersexual agonistic behavior is as strong as intrasexual agonistic behavior. Both types of behavior probably result from competition among individuals of both sexes for a common limiting resource. Several lines of evidence suggest that the limiting resource is nectar produced by C. floribunda. MANY INSECTS ACTIVELY DEFEND resources against use by conspecific individuals. Such territorial behavior, which occurs in at least eight orders of insects (Price 1975 and references), most commonly takes one of two forms. In one form, competition among males for access to females has led to the evolution of mating territories, which are often localized around some resource (e.g., oviposition site) used by females. Normally only males interact agonistically (Campanella and Wolf 1974, Jacobs 1955, Otte and Joern 1975, Ziegler 1972). A second common form of territorial behavior of insects is defense by females of a limited or patchy larval food supply. Usually only females interact agonistically in this type of resource defense (Pukowski 1933). In neither of these forms of resource defense is intersexual competition important. When both sexes compete for the same limiting resource, however, agonistic interactions may be as strong between individuals of opposite sexes as between individuals of the same sex. In this paper we describe resource defense by a soldier beetle, Chauliognathus distinguendus. This beetle is unlike other insects that have been studied in that intersexual as well as intrasexual competition for a limiting resource is intense. Intersexual agonistic interactions are consequently as frequent as those between individuals of the same sex. METHODS During the period 3-11 August 1974, C. distinguendus was common on Coccoloba floribunda Lindau (Polygonaceae) trees growing among the open vegetation bordering a seasonal swamp in Guanacaste Province, Costa Rica. We saw few beetles on other plant species during this period, although they are known to visit flowers of other plants at other times of the year and in other locations (G. Frankie, pers. comm.). The swamp study site was approximately 2 km southeast of the Palo Verde Field Station, which is operated by the Organization for Tropical Studies. Coccoloba floribunda, a small tree up to 5 m tall, is dioecious with erect inflorescences borne singly at the leaf nodes. Each inflorescence contains many small flowers, only a small fraction of which are open on any one day. Chauliognathus distinguendus, a conspicuous yellow and black beetle 13 to 22 mm long, is found from Mexico to Costa Rica (Champion 1914). These beetles frequently confined their activities to a single C. floribunda inflorescence for several hours of the day, and while on an inflorescence, a beetle would continually visit flowers and attempt to feed on the nectar they secreted. We never observed any behavior suggesting that these beetles consume pollen or flower parts. Although beetles were found on both male and female trees, we restricted our observations primarily to beetles on male trees. To facilitate observation of behavior in C. distinguendus, we marked individual beetles with spots of red or blue enamel paint. The markings, which we applied with a toothpick while beetles were on a plant, were irregularly shaped, permitting us to identify each beetle individually. We observed a beetle for 10 to 60 minutes. Every interaction with another beetle was recorded by noting the sex of each beetle, which was made possible by differences in the external genitalia. We also recorded, for every interaction, the reaction of the inflorescence occupant toward the intruder (e.g., no visible reaction, approach without contact, approach and momentary contact, and approach followed by sustained contact), the reaction of the intruder (e.g., immediate movement off the inflorescence, movement off inflorescence after brief 96 BIOTROPICA 11 (2)' 96-10