The aim of this study was to obtain basic information about the ecology of S. aureus on modern swine farms, based on in depth sampling of pigs in two multiple site systems. Samples were collected from the nose, tonsils, feces, and skin of suckling pigs, weaned pigs, and finishing pigs in each of two groups raised in each system, and from the same samples plus the vagina of sows nursing the selected piglets. S. aureus was prevalent on both farms, all age groups of pigs (sows, suckling, nursery, and finishing) and in all anatomical sites, but no methicillin resistant isolates were detected. Highest prevalence was in nasal, tonsil and skin samples (59 – 65%), and lowest prevalence (27%) was in vaginal samples from sows. Diverse spa types of S. aureus were found on both farms and all age groups. S. aureus was also frequently detected in environmental samples, including air, and in people working on the farms. The 3 predominant S. aureus variants encountered have previously been reported as MRSA isolates from pigs in North America. MLST typing (not part of the proposal) was conducted on selected isolates among the predominant spa types, and ST398 and ST9 were the predominant MLST types found. ST398 is the predominant MRSA sequence type found in swine in Europe, and ST9 variants are most common in several Asian countries. As the most common methicillin sensitive S. aureus (MSSA) spa types found on these farms correspond closely with the MRSA variants reported from Europe (ST398), Asia (ST9) and North America (ST5), it is plausible that these variants have been associated with pigs for a long time, while acquisition of MRSA resistance may be more recent. Evaluation of S. aureus subtypes and MLST types across a more representative sample of farms is indicated.