Three regions of serum esterase activity of chickens were differentiated by means of starch gel electrophoresis and appropriate stains. Regions I and II seemed to represent aliesterase while region III apparently represented a cholinesterase. The patterns of zones of region I were classified into three phenotypes named A, B, and AB and the hypothesis was proposed that these are controlled by two codominant alleles named EsA and AsB. The staining intensity of these esterase zones among adults was sex-associated in that the mean staining intensity of zones was greater in male than in female sera. Frequency of the EsB allele was 0.70, 0.76, and 0.85 in three meat lines.