A helminthologic study was carried out of 100 sheep and 79 weaned lambs affected with strongylatosis, originating from 90 communities. Ovoscopic, clinical, and epizootiologic investigations were conducted with the infected flocks. It was found that these infections were caused by a number of Strongylata species belonging to 8 genera. Two forms of the disease were observed-enzootic and sporadic. They were found to run their course in strict correlation with the season. In weaned lambs the enzootic diseases started in July, had a peak in September, and receded late in the autumn and winter. In adult sheep enzootic outbreaks were observed in November and December. Most numerous were the cases in winter, reaching a peak value in the month of March. Insufficient feeding was shown to be a predisposing factor contributing to the clinical manifestation of strongylatosis. With lambs worming should be carried out prophylactically three times-in June, September, and December, while the sheep it should take place once-in December or January.