Protothecosis was described in many animal!;: with bovine mastitis bei
ng the main form. The increasing number of isolations of Prototheca sp
p. from bovine mastitis cases indicates the need of a detailed evaluat
ion of this problem. Besides this, these algae do not respond to treat
ment with the antimicrobians most frequently applied, leading to elimi
nation of the affected animals, as the best method to control the dise
ase. In two dairy farms in the State of Sao Paulo, Brazil, a total of
155 lactating cows and 52 dry cows were examined. Milli samples were a
septically collected from lactating cows with clinical or subclinical
mastitis. From dry cows, secretion samples from all quarters were coll
ected. All samples were then taken to microbiological exams. At dairy
farm 1, Prototheca spp. was isolated from 14.95% milk samples from lac
tating cows, (all of them cases of subclinical mastitis), and from 8.0
6% samples from dry cows. At dairy farm 2, Prototheca spp. was isolate
d from 5.1% milk samples and there were ten cases of clinical mastitis
due to this agent. Although clinical mastitis has been considered the
main form of occurrence of this pathology until now, it is important
to consider Prototheca spp. as subclinical mastitis pathogen. Protothe
ca zopfii was the main specie isolated. In this study a high rate of i
ntramammarian infections, as clinical or subclinical mastitis during l
actation as well as in dry period, was observed, high lighting the imp
ortance of suitable diagnosis,so that control and preventive measures
can be implemented to avoid dissemination of the agent.