The epidemiology and importance of coagulase negative staphylococcal (CNS) mastitis in Prince Edward Island had not been documented. To investigate this, a cohort of 84 cows at seven farms were quarter sampled eight times over a lactation, commencing with samples taken prior to drying off in the previous lactation. Thirteen species of CNS were isolated. The quarter prevalence of CNS mastitis varied from 4.8% to 6.4% in the first five months of lactation and increased to 14.2 to 16.6% in the last four months of lactation. The geometric mean somatic cell counts (SCC) for quarters infected with CNS and uninfected quarters were 90 x 10(3) and 64 x 10(3) respectively (difference significant at p > 0.005). The two month new infection risk of CNS was 9.0% while the two month elimination risk was 74.4%. Infection with CNS did not alter the risk of subsequent infection with Staphylococcus aureus. The results from this project support the classification of CNS as a minor pathogen in mastitis control programs.