A three-year survey to determine the incidence of clinical mastitis and the associated bacteria in dairy herds in England and Wales is described. Escherichia coli was the predominant organism in each year. Streptococcus uberis and Staphylococcus aureus were important for part for each year. The annual incidence of mastitis declined from 54.6 cases per 100 cows in 1980 to 41.2 in 1982. The incidence increased with age and declined with increasing herd size. The culling rate due solely to mastitis was 3 per cent. Strep uberis was the pathogen most frequently isolated from clinical cases which occurred in the dry period. Thirty per cent of all cases recurred at least once and staphylococcal cases exhibited the highest frequency of recurrence.