Abstract Babesia canis and Babesia gibsoni were found to be serologically related on the basis of cross-reactions between corpuscular and soluble antigens derived from these organisms and sera of dogs recovered from acute infections caused by each of the parasites. By means of a gel-precipitation (GP) test each Babesia organism possessed at least one species-specific antigen. This antigenic difference was also indicated by the indirect hemagglutination (IHA) test. In the capillary-tube agglutination (CA) and indirect fluorescent antibody (IFA) tests, which used corpuscular antigens of these organisms and undiluted sera, serologic differences between the two organisms was not detected. The time of appearance of antibody detectable in the IHA test was closely correlated with the onset of the disease and appearance of the parasites in the peripheral blood. Based upon results obtained with sera of artificially and naturally infected dogs, the IHA test appears useful for detection and titration of antibodies in canine babesial infections.