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Corn (Zea mays L.) plants were assayed for nitrogenase activity (C(2)H(2) reduction) during early ear development. Hybrid corn and inbred lines were grown separately at two experimental fields in New Jersey. Acetylene-dependent ethylene production was observed a few hours after harvest, from the field, on intact plants, root-soil cores, lower stem segments, and excised roots, all assayed under air and not preincubated previously. Incubation of excised roots at 1% O(2) resulted in lower rates of C(2)H(2) reduction. The time course of C(2)H(2) reduction by excised roots, assayed in air, was similar for all genotypes studied (two hybrids, eight inbreds, and a cross of corn x teosinte) and indicated that a long preincubation at reduced O(2) is not absolutely required for early detection of nitrogenase activity. Isolation of N(2)-fixing bacteria from within the roots and stems, together with the diurnal fluctuation of nitrogenase activity in response to day/night cycles, were indicative of a close association with plant function. Collectively, the results provided strong evidence for the occurrence of nitrogenase activity associated with corn plants growing in a temperate climate and dependent upon indigenous N(2)-fixing bacteria.