The hypothesis tested was that dairy herds which import cattle for replacement or expansion have a higher prevalence of Staphylococcus aureus mastitis and a greater number of new Staphylococcus aureus strains enter their herds than closed herds. Fifteen commercial dairy herds were divided into four groups based on cattle importation practices. Composite foremilk samples were collected at 4-monthly intervals for 1 year from all lactating cattle. Additionally, foremilk samples were collected from cattle at parturition and skin swabs were taken from the udder of primiparous heifers. All samples were cultured for Staphylococcus aureus and isolates were strain-typed using pulsed-field gel electrophoresis. Herds that purchased replacement heifers had a higher prevalence of Staphylococcus aureus mastitis than herds that purchased lactating cattle for expansion (P = 0.02). Herds that purchased replacement heifers had more total strains of Staphylococcus aureus (P = 0.01) and more new strains (P = 0.04) enter the herd than closed herds.