Five hundred herds were examined to determine the prevalence of subclinical mastitis in the British dairy herd. The prevalences of the various infections recorded were Streptococcus agalactiae 3.4 per cent of quarters, S dysgalactiae 1.1 per cent, S uberis 1.5 per cent and Staphylococcus pyogenes 8.1 per cent. There were regional differences in the prevalence of some of these pathogens. The national prevalence of subclinical mastitis as defined by the International Dairy Federation was 9.6 per cent of all quarters. Udder infections were less prevalent in herds where mastitis control measures were being used. The prevalence of infection also declined as the size of herd increased. However, as the uptake of mastitis control measures was greater in the larger herds it seems likely that the widespread use of control measures was the most important factor responsible for the relatively low prevalence of subclinical mastitis.