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The proportion of different strains of Staphylococcus aureus was tested in four groups of lactating dairy cows in six herds with a high overall prevalence of Staph. aureus using random amplified polymorphic DNA PCR. Group 1 included primiparous cows in early lactation (<50 days in milk, DIM). Group 2 consisted of primiparous cows in late lactation (>250 days in milk). Groups 3 and 4 were multiparous cows in the respective stages of lactation. Eight cows from each group on each farm were tested. Overall quarter prevalence of Staph. aureus ranged from 23.4 to 32.0% in the herds. Of the 130 isolates included in the analysis 86.9% were high prevalence strains (more than three isolates per herd), while 13.1% were strains that were only identified in one or two samples. Low prevalence strains were found in all six herds. The proportion of low prevalence strains was higher in multiparous than in primiparous cows (odds ratio, OR 4.4, 1.2-16.6). It is concluded that low prevalence Staph. aureus strains are common even in herds with a high prevalence of Staph. aureus and that their frequency is lower in primiparous cows than in older cows.