Fixed nitrogen is a limiting nutrient in most terrestrial ecosystems and has been assumed to be supplied almost entirely by free-living bacteria as well as by bacteria living in association with plants. The survival and growth of many arthropods on diets with extremely high carbon to nitrogen (C:N) ratios suggest that these arthropods are not obtaining sufficient nitrogen from their diets but must be obtaining additional nitrogen from some other source(s). Estimates of N(2) fixation have suggested that symbiotic microbes of some arthropod hindguts could be obtaining this additional nitrogen as a result of nitrogen fixation. With the recent availability of antibody and nucleic acid probes, the presence of the enzyme that reduces dinitrogen gas to ammonia (nitrogenase) as well as the presence of its transcripts can be detected and localized with great sensitivity. A preliminary survey of a few detritivores indicates that nitrogen-fixing microbes of diverse forms are widespread in arthropod hindguts. In calculating nitrogen budgets, the possible contributions of nitrogen fixation by symbionts in arthropod guts, however, has been largely ignored. N(2) fixation in arthropod guts, with rates as high as 10-40 kg/ha/year being possible, may represent a significant contribution both to the growth of arthropods and to their ecosystem functions of processing carbon and nitrogen.