Milk samples were collected from 180 heifers from 20 herds around parturition to study the prevalence and persistence of intramammary infection (IMI). A total of 3382 quarter milk samples was collected each week from 4 wk before parturition to 4 wk after parturition. Staphylococcus chromogenes, the most commonly found bacterial species before parturition, was isolated from 15% of all quarters. However, IMI with Staph. chromogenes decreased shortly after parturition to around 1% of quarters. Infectious with Staphylococcus simulans and Staphylococcus epidermidis occurred in 1 to 3% of quarters both before and after parturition. Infections with Staph. simulans persisted in the same quarter for several weeks, but IMI with Staph. epidermidis were transient. Streptococcus dysgalactiae was isolated from 4 to 6% of quarters before and immediately after parturition, but the prevalence of IMI with Strep. dysgalactiae decreased. A strong association existed between IMI with Strep. dysgalactiae before parturition and IMI with Strep. dysgalactiae after parturition, and, using DNA typing, the persistence of 9 IMI was confirmed. Infections with Staphylococcus aureus rarely occurred before parturition, but the rate of IMI with Staph. aureus greatly increased during the 1st wk after parturition. No association was found between bacterial status before parturition and IMI with Staph. aureus after parturition. Overall, the presence of bacterial IMI in a quarter before parturition increased the risk of IMI for the lactating cow. This study demonstrated that variability in prevalence and duration of IMI according to bacterial species occurred around first parturition.