An epidemiological study on gastrointestinal tract nematodiosis of small ruminants in three different agro-ecological zones of southern Ethiopia was carried out from August 2004 to April 2005 on 180 gastrointestinal tracts and 2,828 faecal samples collected from sheep and goats to identify nematode species, determine the occurrence and factors related to the nematode infection. The study was carried out through post-mortem examination and coprological examinations using floatation and McMaster egg counting methods. Post mortem examination revealed the existence of ten (10) nematode species with different percent prevalence rates (Oesophagostomum columbianum, 92.2; Trichostrongylus colubriformis, 73.9; Haemonchus contortus, 68.9; Trichostrongylus axei, 61.7; Tricuris ovis, 42.8; Bunostomum trigonocephalum, 18.9; Teladorsagia spp., 15.6; Trichostrongylus probolurus, 5.0; Strongyloides papillosus, 1.1 and Cooperia curticei, 0.5). The identification of Teladorsagia spp. and Trichostrongylus probolurus was new to the region and even to the country after Graber, (1975) and the identification of Cooperia curticei was for the first time in Ethiopia. The prevalence of gastrointestinal tract nematodiosis varies from 98.3% in lowland and midland agro-ecology to 100% in highlands, with an overall prevalence of 98.9% in all agro-ecological zones. The variation in the prevalence of individual nematode species on agro-ecological basis was significant (P0.05). The seasonal point prevalence of Haemonchus contortus, Trichostrongylus axei, Bunstomum trigonocephalum, Oesophagostomum columbianum, Trichuris ovis and Trichostrongylus colubriformis were significantly varied (p< 0.05) between seasons of the year. Mean nematode burden of different seasons of the year have shown significant differences for seasonal variation (p