An outbreak of subclinical mastitis in a flock of 620 milking sheep was investigated. Microbiological and epidemiological analyses identified the causative agent as belonging to the Burkholderia cepacia complex (formerly Pseudomonas cepacia). Every ewe in the milking flock was individually tested for subclinical mastitis on two separate occasions, 6 weeks apart, by the California (rapid) mastitis test (CMT). The proportion of CMT-positive ewes was 69 of 393 (17.6%) on the first sampling and 27 of 490 (5.5%) on the second sampling. Pure B. cepacia cultures identified with the API 20 NE system were grown from 64 of 96 (66.7%) CMT-positive ewes and from 1 of 33 (3.0%) CMT-negative ewes. Statistical analysis confirmed the significant association between a positive CMT result and a positive culture result for B. cepacia complex. Additional polyphasic taxonomic analyses of eight isolates showed that seven belonged to B. cepacia genomovar III; the remaining isolate was identified as Burkholderia vietnamiensis (formerly B. cepacia genomovar V). Bacteriological investigation of samples from milking equipment and other environmental sites failed to identify "B. cepacia" in any of the samples taken. To our knowledge, this is the first report of an outbreak of natural infection in animals caused by B. cepacia complex and the first description of B. cepacia complex infection in sheep.