Arthropods were collected for 239,240PU (239Pu) and 238Pu analysis from three study plots in close proximity to the Rocky Flats nuclear weapons plant and from a site 110 km N-NE of the plant. Mean 239Pu concentrations in arthropods were 265, 16, 0.7 and 0.5 dis/min g−1 at the three Rocky Flats study plots and at the control site, respectively. Arthropod 239Pu concentration data were statistically analyzed by season of collection, taxonomic group, and sampling site. Only the collection site differences were significant (a = 0.01) and these were correlated with 239Pu concentrations in soil. The mean activity ratio of 239Pu to 238Pu in arthropods was 52, similar to the value of 51 obtained for soil. The mean ratio of 239Pu in arthropods to 239Pu in 0–3 cm soil at Rocky Flats was 9 × 10−1. Arthropod biomass and Pu concentration data indicated that only about 10−8 of the total plutonium inventory is in the arthropod component of the ecosystem. Leafhoppers, grasshoppers and spiders accounted for roughly 80% of the arthropod inventory of 239Pu.