The individual and social behavior of 73 captive opossums, Didelphis marsupialis virginiana, was studied. The patterns of behavior seen in Didelphis appear generalized, correlating with the opossum's primitive morphology and taxonomic status. Diverse locomotor patterns, arboreal modifications, and highly unselective food habits explain in part the diversity of habitats frequented by Dudelphis in North America. A well developed attitude of passive defense is seen in interspecific encounters and the adaptive significance of such behavior is considered. Social behavior is poorly developed and, with the exception of mating, most intraspecific encounters between adults are agonistic in nature. Although marsupials are usually not successful in competition with placentals, the opossum is an obvious exception. Presumably, its generalized behavior has allowed Didelphis to exploit a variety of food sources and habitat types. This, together with a high reproductive potential, may account for the evolutionary persistence of the species, and its current ecological success. INTRODUCTION The opossum, Didelphis marsupialis (Family Didelphidae) is the only marsupial native to the United States. Morphologically, Didelphis resembles members of the Upper Cretaceous didelphid stock which underwent adaptive radiation in South America and from which all extant marsupials evolved (Simpson, 1945; Clemens, 1968). Although marsupials are usually not successful in competition with placentals, Didelphis is notable for its persistence, particularly in the eastern United States (Hamilton, 1958). This success may be due, in part, to the opossum's generalized morphology (Haines, 1958); but other aspects of its biology must also be involved. Despite the unique taxonomic position of Didelphis, little attention has been given to the details of its behavior. With the exception of studies on the ontogeny of behavior in pouch young (Langworthy, 1925, 1928), on reproductive behavior (Reynolds, 1952), and on defensive behavior (Francq, 1969), most information on the behavior of the opossum is of an anecdotal nature. The present study attempts to provide an account of the behavior of the opossum, with the specific objective of learning the extent to which behavioral and associated structural features may have contributed to its success. In addition, the primitive status of the opossum makes its behavior of exceptional interest from the broader standpoint of the origin and evolution of mammalian patterns of behavior in general. METHODS AND MATERIALS Opossums were captured in the vicinity of Ithaca, Tompkins Co., New York. A total of 76 individuals (33 males and 43 females) of the 1 Present address: Department of Biology, Fairleigh Dickinson University, Madison, New Jersey 07940.