Mastitis due to Prototheca zopfii was diagnosed in three of 28 cows in a dairy herd. As two cows continued to shed algae after 45 days they were slaughtered and organs were examined by cultivation, histology, immunohistochemistry and electron microscopy. Algae were restricted to the mammary glands and regional lymph nodes in which a granulomatous inflammation was seen. Algae were predominantly seen in macrophages but neutrophils also contained organisms. In macrophages both sporangiospores and sporangia were found, suggesting that intracellular proliferation may be responsible for the failure to overcome the infection. Serum samples from all cows were assayed for antibodies against P. zopfii in an enzyme-linked immunosorbent assay (ELISA). Although the highest titre was found in an infected cow the difference between the mean values of the titre in infected and non-infected cows was not significant.