Relatively large parts of the Slovak territory are marginal regions, which in terms of the economy of ruminant rearing can effectively produce animal commodities only occasionally. The aim of this study was to evaluate the occurrence and aetiology of mastitis during one milking season in two sheep dairy herds of 224 (A) and 270 (B) ewes situated in the marginal parts of Slovakia. A complex examination of health status of ewes based on: the clinical examination of the udder, macroscopic evaluation of milk with the California mastitis test (CMT) and the bacteriological analysis of raw milk samples from individual halves were carried out at the beginning (April) and at the end (September) of the milking season. The prevalence of intramammary infection (IMI) in the herds of ewes (A and B) were 19.1 % and 14.3 %, respectively. In both herds we confirmed predominantly the subclinical forms of IMI. The highest percentages of aetiological agents were determined for coagulase negative staphylococci (CoNS), such as Staphylococcus chromogenes, Staphylococcus schleiferi, Staphylococcus epidermidis, Staphylococcus xylosus, Staphylococcus piscifermentans and Staphylococcus intermedius, the occurrences of which were determined in 41 (48.2 %) and 37 (47.9 %) positive milk samples, respectively.