Effectiveness of eprinomectin, which until now has not been assessed in Turkey, was tested against gastrointestinal nematodes of naturally infected cattle under field conditions in Adapazari, Sakarya. Out of 25 cattle used in trial, 17 were treated with eprinomectin and 8 were kept as untreated controls. To determine gastrointestinal nematode fauna, a fecal culture was prepared from the mixed fecal samples of 25 trial cattle at 26-27 degrees C for 3 weeks. Eprinomectin 0.5% pour-on (Eprinomec 0.5% Dokme Cozelti (R), TOPKIM) at the recommended dose of 1 mL/10 kg bw (0.5 mg active ingredient/kg bw) was applied along the backline of each animal (day 0). It was determined that the gastrointestinal nematode fauna of 25 trial cows comprised Nematodirus (17%), Strongyloides (9%), Trichostrongylus (24%), Ostertagia (20%), Haemonchus (12%), Cooperia (10%), Oesophagostomum (6%), and Bunostomum (2%) genera. A t-test for independent samples showed no significant difference (P > 0.05) between the log (EPG + 1) values of control and treatment groups on the treatment day (day 0), whereas the differences were significant (P < 0.001) on days 3, 10, and 17. Fecal egg count reduction percentage (effectiveness) was > 99.5% according to geometric means and > 97.3% according to arithmetic means. However, effectiveness against individual nematode species may differ somewhat from these percentages. No adverse reactions were observed in cattle treated with eprinomectin.