This paper describes an outbreak of subclinical mastitis caused by Pseudomonas aeruginosa in a dairy herd. An increase in the bulk milk somatic cell count from 82 000 to 327 000 cells/ml was observed over a period of one month. P. aeruginosa was diagnosed as the causal agent of the problems. All infected cows were identified using bacteriological culture of milk samples. As the farmer explicitly asked for a treatment, disinfection and renewal of the teat liners were carried out and all infected quarters and cows were treated systemically and intramammarily with a cephalosporin antibiotic. Treatment was unsuccessful and all persistently infected cows were culled, after which the bulk somatic cell count returned to a normal level.