Arthropod populations in the understory of a Costa Rican, lower-montane rain forest were monitored periodically for 19 mo. by sweep sampling and Malaise trapping. Abundance fluctuated three-fold during the study and was highest in the late dry season and early rainy season, April-June, and lowest during the cooler, windy months of Nov.-Jan. High species diversity (H'' and .alpha.) in Coleoptera coincided with peak abundance. Average body size in the whole fauna was largest in April, when arthropod numbers were high. Peaks in abundance of some predatory groups lagged behind peaks in other arthropod groups. The annual cycle appears to be composed of a series of short-lived, seasonal subfaunae. Results are discussed relative to plant productivity, climatic factors and findings from other locations.