Summary The relative frequencies and genetic determination of the two electrophoretically distinguishable adult bovine hemoglobin types were investigated in a large sample drawn from a number of different breeds and sources. Electrophoretic studies of hemoglobins from 121 males and 646 females, representing nine cattle breeds, indicate that the Ayrshire, Holstein, Angus, Hereford, and Milking Shorthorn breeds probably have only Type A hemoglobin; whereas, the Brown Swiss, Guernsey, Jersey, and Sindhi breeds have both Type A and Type B. An analysis of hemoglobin types among progeny from parents of known types substantiates the theory that the occurrence of these two types of hemoglobin is genetically determined by a pair of codominant alleles—HbA, determining Type A, and HbB, determining Type B. Estimates of the frequeney of HbB among the major dairy breeds are: Ayrshire—0.000, Holstein—0.000, Guernsey—0.116, Brown Swiss—0.117, Jersey—0.333. No gross physiological differences have been observed to be correlated with differences in hemoglobin type. The trait may be of some practical value in parentage determination and in the tracing of breed origins. The most significant feature of these results is that they represent a further demonstration of single-gene determination of a protein specificity.