Vertical distribution patterns and seasonal changes in abundance of the microarthropod fauna of a ponderosa pine forest soil are described. The study area, located near Grass Valley, California, is marked by an abundant rainfall from October to May, and neardrought conditions from June to September. Microarthropods in the study area, estimated at about 220,739 per square meter, were characterized by considerable diversity. The Acarina (130 species) were the most abundantly represented—with densities of about 146,107 per square meter; the Prostigmata were dominant. The Collembola ranked next (44,039) per square meter. Smaller proportions of other arthropod groups were found—including the Psocoptera, Protura, and Pauropoda. Abundance of these groups and of various subcategories of Acarina and Collembola are presented. Substantial proportions of the microarthropod fauna occurred and remained fairly constant in the mineral subsoil all year. Greater densities in the humus and litter layers during the wet season was due mostly to upward shifts of the fauna. Psocoptera, Bdelloidea, and Raphignathoidea, however, remained primarily in the litter and humus all year becoming more abundant during the dry season. Other surface-dwelling forms showed little seasonal change in abundance.