As a collaborative project between the Organization for Tropical Studies (a consortium of 45 U.S. Costa Rican universities and museums) and the National Biodiversity Institute (INBio, a Costa Rican nongovernmental organization), a biodiversity inventory of tropical rainforest insects, spiders, and mites will initiated for La Selva, a biotically rich and scientifically active biological station situated in the Atlantic lowlands of Costa Rica. This stage of the project aims to produce full, species-level inventories for several focal groups of insects, spiders, and mites, including some of the most important groups in rainforest habitats. These inventories will be based on ecologically structured, repeatable sampling methods, as well as on less- structure supplemental collecting. A must broader range of survey groups, each including a focal group, will be inventoried using the structured sampling techniques alone. Statistical methods will be explored for estimating the total number of insect, spider, and mite species in the study areas, using the full inventory data for the groups to evaluate these methods and to calibrate estimates of species number for the broader survey groups. The sampling design will permit evaluation of the contributions of habitat would type vegetation structure species, and season to overall species richness. These estimated promise to provide the most realistic and comprehensive data yet available on the diversity of insect and spider species of tropical rainforests, and diversity estimates for any rainforest mite fauna. These groups or organisms, while representing well overall the species on earth, are still among the most poorly known. Specimens will be integrated into the national biodiversity inventory already underway at INBio, thus placing the La Selva survey material in a larger geographical and biological context, promoting further study of this material by INBio's growing international network of specialists, and ensuring permanent accessibility of the specimens. The project is structured to promote extensive new avenues of collaboration between North American and Costa Rican scientists.