JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology. Abstract. Wide-ranging but ecologically variable species and their close relatives provide the best opportunity to analyze the role of trade-offs in specialization. Poecilochirus carabi Can. is a holarctically distributed mite that rides between patches of carrion on the bodies of carrion beetles (Silphidae, Nicrophorus). Using behavioral tests, we demonstrated differences in local host specialization of P. carabi at two sites in Michigan, the Kellogg Biological Station (KBS) and the University of Michigan Biological Station (UMBS). At KBS, Nicrophorus orbicollis and N. tomentosus each supported a strict host specialist. These KBS "orbicollisand tomentosus-specialists" differed significantly in both body size and a measure of dorsal setal lengths. Laboratory crosses between specialists were not successful, suggesting that the specialists are distinct species. At UMBS, in contrast, such strong specialists were not found. Mites preferred two beetle species, N. tomentosus or N. defodiens (a species not found at KBS), over all others, but moved between these two species readily. UMBS mites all resembled the KBS tomentosus-specialist in morphology. No orbicollisspecialist mites were found at UMBS, despite the high relative abundance of N. orbicollis in the beetle community.We raised mites from each site on all available hosts to test whether preference for a host species was associated with higher fitness in association with that host species. Although P. carabi were able to reproduce in association with any Nicrophorus species, mites at UMBS had the highest reproductive success on the two species that they preferred. Each KBS specialist mite was raised in association with N. tomentosus and N. orbicollis. While the tomentosus-specialist had significantly higher reproductive success in association with its preferred host species, the orbicollis-specialist showed no difference in reproductive success between the two host species. However, when the pattern of dispersal on beetles was considered, a difference in fitness between the two hosts was apparent. This difference in dispersal pattern was due to an interaction between the parental behavior of the two host species and mite generation time. We discuss the evidence for trade-offs in this system and the use of interspecific comparisons in the analysis of specialization.trade-offs in the evolution of specialization (Futuyma and Moreno 1988, Via 1990). This approach avoids many pitfalls associated with comparisons between species, but also suffers from a lack of power to test the role of specialization in speciation (Rausher 1988...