A study was initiated to (1) determine species of gastrointestinal nematodes present in cattle in selected areas of Oregon, (2) examine seasonal abundance of these nematodes as an indicator of periods of transmission, and (3) determine when developmental inhibition occurs in the major genera of nematodes encountered. Four study sites were chosen: Corvallis, Langlois, near Fort Rock, and southeast of Klamath Falls on the Oregon-California border. Eight sets of three to four tracer calves each were introduced onto pasture at each site over a 2 year period with time of turn out onto pasture corresponding to late spring, mid-summer, late fall and late winter. At Corvallis, a distinct seasonality in parasite transmission was evident, peaking during the fall and winter. At Langlois, transmission was fairly constant throughout the year. No discernible patterns were evident at either Klamath Falls or Fort Rock. Eight nematode genera were found during the study. Ostertagia, Cooperia, Nematodirus and Trichostrongylus were present at all study sites and were the most common genera at each. Trichuris was found at all sites except Klamath Falls. Oesophagostomum was present in tracers only from Langlois and Corvallis while Haemonchus was found only at Klamath Falls and Fort Rock. Capillaria was only present at Klamath Falls. Where possible, specific transmission patterns for Nematodirus, Cooperia, and Ostertagia were determined for each site. Nematodirus was transmitted fairly steadily at both Langlois and Corvallis but was quite variable at Fort Rock. Developmental arrest was detected in this genus at all study sites during the fall and/or winter. Cooperia exhibited the most seasonally defined pattern of transmission, peaking during the fall and winter at Langlois, Corvallis and Klamath Falls. Hypobiotic larvae of Cooperia were present during the fall and/or winter only at Langlois and Corvallis. Peak transmission of Ostertagia at Langlois and Corvallis occurred during the fall and winter. At Fort Rock, transmission was lowest in the fall and increased in the winter. Hypobiotic larvae were evident in the fall and winter at Corvallis, Fort Rock and Klamath Falls. These data suggest Type II ostertagiasis may occur in late winter through spring in these areas. Hypobiotic larvae of Ostertagia were not detected at Langlois. The lack of appropriate environmental stimuli is one possible explanation for the apparent lack of hypobiosis at that site.