Two tropical papilionid butterflies, Battus polydamus and Battus bellus, maintain stable adult numbers at one major food source (flowers of Stachytarpheta jamaicensis; Verbenaceae) over several months in the premontane tropical wet forest region of northeastern Costa Rica. Adults of both species enjoy high survivorship from predation, presumably as a result of Miillerian mimetic association at this food source. At least one of these species, Battus polydamus, exists as small, isolated breeding populations whose distribution is partially dependent upon the location of patches of Aristolochia (Aristolochiaceae), the larval host plant. Lack of well-developed dispersal ability (vagility) in these butterflies and their patchy distributions in tropical habitats place supreme selective advantage on the evolution of a Miillerian association; adult survivorship at low densities is lengthened by mimicry, thus postponing or preventing local extinctions of isolates. IT IS PERHAPS somewhat surprising that despite the intriguing results of laboratory studies on the palatability (acceptability) of neotropical butterflies (e.g., Brower, Brower, and Collins 1963; Brower and Brower 1964) and their implications for the theory of mimicry, there have been relatively few attempts to examine such systems of resemblance in natural populations. The notable exceptions to this, of course, are studies on the relative abundances of models and mimics in North American mimicry complexes, notably the Danaus plexipps Linn.-Limenitis archippus Cram. mimicry association (Brower 1958a, b), and the Battus philenor Linn. mimicry complex (Brower 1958c, Brower and Brower 1962). It is clear that both Batesian and Miillerian mimicry complexes of butterflies are most abundant in the tropics (Darwin 1893, Poulton 1890, Fisher 1930, Turner 1965, 1970), and, therefore, studies on the verification of the role of unpalatability in mimicry theory can be best investigated in the tropics. Both descriptive and experimental field studies in the tropics are needed to understand the adaptive significance of effective mimicry in natural populations. This paper describes a Miillerian mimetic association between two species of tropical papilionid butterflies, namely, Battus polydamus polydamnns Linnaeus and Battus bellms varms Koller (fig. 1), in the northeastern Caribbean lowlands of Costa Rica. In light of the findings reported herein on an apparently high cohesiveness in co-occurring local adult populations of both butterflies and also from previous study of the population biology of Battus polydamus (Young 1972a), the hypothesis is advanced that natural selection has favored the maintenance of numerically stable adult complexes of these species at feeding sites as one means of ensuring their local survivorship under conditions of very low population densities -and the highly patchy distribution of larval host plants. Underlying this hypothesis is the assumption that individuals of both species are highly unacceptable as prey, since their larvae feed primarily upon the alkaloid-rich tropical vines belonging to the genus Aristolochia (Aristolochiaceae). Previous studies (Brower 1958b, Brower and Brower 1964) have established the fact that Aristolochia-feeding butterflies of the genera Battus and Parides are highly unacceptable as prey for caged birds. As revealed in the present field study, there is -an apparent high survivorship of adults for both species where they co-occur at flowers, suggesting that avian predation is in fact negligible in local mimicry complexes of Battus polydammus and Battus bellus. Similar observations on adults belonging to two species of Parides in northeastern Costa Rica (Young 1972b) indicate that avian predation is generally low in these other Aristolochia-feeding butterflies. However, as recently shown by Cook et al. (1971), such butterflies may be killed in large numbers by heavy rains at the commencement of the wet season in Trinidad.