The genera and species of bovine gastrointestinal (GI) nematodes were recorded from 1995 to 1997 throughout four seasons, in three different humid, wet, subtropical locations, i.e.: Sauta, Medellin, and Tizimin (Mexico's states of Nayarit, Veracruz, and Yucatan, respectively). In each location, five freshly weaned, GI nematode-free calves were placed on grasslands, during four time periods. Placements were performed as follows: a) mid raining season (August), b) late raining season (November), c) early dry season (February), and d) early raining season (June). After a 30-day, free-range period cattle were slaughter and adult GI nematodes were collected and identified. Means and standard deviations of nematode species were as follows: Sauta, Haemonchus contortus 32+-8, 615+-115, 267+-87, and 20+-4, Bunostomum phlebotomum: 5+-1, 2+-2, 2+-2, and 0, Oesophagostomum radiatum: 84+-22, 46+-21, 76+-39, and 22+-14, and Trichuris ovis 8+-1, 104+-21, 92+-35, and 112+-38, Medellin, H. contortus 326+-152, 613+-210, 1167+-387, and 12+-8, H. similis: 30+-21, 56+-22, 35+-16, and 0, Cooperia punctata: 320+-185, 414+-409, 47+-28, and 0+-0, B. phlebotomum: 10+-7, 2+-2, 5+-2, and 0, O. radiatum: 106+-42, 228+-61, 72+-39, and 228+-144, Tizimin, H. contortus: 160+-58, 440+-380, 1514+-718, and 300+-184, C. punctata: 46+-31, 90+-84, 930+-1180, and 55+-34, O. radiatum: 4+-4, 26+-33, 0, and 7+-7, for the animals placed in August, November, February, and June, respectively. Significant differences (P<0.05) were found among the three locations. Genera and species varied in terms of both presence, and quantity.