In a field investigation of ovine mastitis in eight flocks in Southern Greece, a population of 760 dairy ewes was monitored during lactation. The prevalence of subclinical mastitis was 4.5% on the 1st sampling (at weaning of lambs), 11.2% on the 2nd sampling (2 to 3 weeks later) and 16.9% on the 3rd sampling (6 to 8 weeks after the second sampling). Although the prevalence of subclinical mastitis increased as the lactation period advanced, the risk rate of the disease decreased. Coagulase-negative staphylococci ( Staphylococcus epidermidis, Staph. simulans, Staph. chromogenes, Staph. xylosus) were the main causal agents of the disease, but Staph. aureus, streptococci, Bacillus spp., Escherichia coli, Pasteurella haemolytica and Actinomyces pyogenes were also isolated from cases of subclinical mastitis. Of the 1084 milk samples collected, somatic cell counts more than 1.0 × 10 6 cells/ml were found in 97 (8.9%); 64 were bacteriologically positive, but 33 bacteriologically negative.