Animal-infective forms (metacyclics) of Trypanosoma brucei brucei were grown in continuous culture at 29°C from the salivary glands of infected tsetse. The parasites were found by immunofluorescent staining to be antigenically stable in the course of cultivation. When the antigenically stable parasites were incubated at 37°C they transformed from long and thin forms to short and stumpy ones and finally to long and slender parasites. The latter generated antigenic variants starting from the second, third or fourth week of incubation at 37°C. Once antigenic variants appeared at 37°C it was not possible to restore the parasites back to antigenically stable populations when they were transferred back to 29°C. These findings may have far-reaching implications in the study of antigenic variation in pathogenic African trypanosomes and the possible development of immunizing reagents against tsetseborne trypanosomiasis.