Morbidity due toCorynebacterium pseudotuberculosisinfection occurred in 29 dairy herds. The disease appeared basically in three clinical forms: cutaneous, mastitic, and visceral. The appearance of the disease showed a marked seasonality: in 23 herds it occurred during the spring and summer months (dry season) (March‐October). The mastitic form occurred in only 10 herds and the causative bacterium was isolated from 33 cows (5.8%). All the strains of C.pseudotuberculosisisolated from the milk samples were found not to be nitrate reducers. The bacterium was excreted in the milk of six cows from herd B during a period of 11 months. In the mastitic cows, a decrease in milk production and considerable increases in the somatic cell count were noted.