Man and higher vertebrates have been shown to be sensitive to carbon monoxide (CO) in concentrations as low as 100 ppm. It is the objective of this study to learn whether similar concentrations have deleterious effects on forest litter invertebrates. The species selected are among the fauna which play an important role in soil structure and in the recycling of energy fixed in plant and animal tissue. With CO being carried into the atmosphere in increasing quantities by automobile emissions, what effects might be expected on these animals? The animals studied were Enchytraeus sp., Helodrilus tenuis (Oligochaeta), Arion fasciatus (Gastropoda), Armadillidium vulgare, Trichoniscus demivirgo, Tracheoniscus rathkei (Isopoda), Spirobolus americanus, Diploiulus sp. (Diplopoda), Leiobunum calcar and L. politum (Phalangida).