SUMMARY In spite of numerous attempts to define Streptococcus uberis by biochemical and serological methods, and more recently by nucleic acid hybridization techniques, there remains a considerable area of disagreement as to which strains should be included in the species and even whether the species itself should be further divided. This confusion is no doubt responsible in part for the variation in the reported incidence of Str. uberis mastitis. Surveys of the prevalence of intramammary infection over the last 20 years reveal that while Staph. aureus is consistently the most common cause of bovine mastitis, the prevalence of Str. agalactiae is decreasing because of the introduction of control programmes and use of antibiotic therapy. Also, the percentage of animals infected with Str. uberis has remained constant. This suggests that while mastitis control systems have done little to reduce the prevalence of Str. uberis mastitis among lactating cows there is little indication that the incidence has increased with the decline of the more common pathogens. Although seldom causing acute mastitis, Str. uberis infections often remain subclinical for long periods and in the absence of treatment can result in a serious loss of milk yield. Infection, like that of the other major mastitis-causing organisms, is not influenced directly by the season of the year, the lactation age of the cow or by breed, but it is the predominant cause of mastitis in unmilked cows. Str. uberis differs from the other mastitis-causing streptococci and Staph. aureus in that it can be isolated from many sites on the body of cows in the absence of intramammary infection, and it has been suggested that at least some of these sites may act as a reservoir of infection. However, the organism has seldom been found in large numbers anywhere other than in the mammary gland and there is little evidence of either colonization or pathogenic activity anywhere other than in the udder.