Abstract: The relation between the frequency distribution of serum prealbumin phenotype and the egg-laying capacity has been determined by vertical poly-acrylamide-gel electrophoresis for different families of the putian black duck.The Putian black duck has five phenotypes of serum prealbumin:(1) Pal-1, with two prealbumin bands of similar thickness in electrophore-gram, one at the anode, the other at the albumin end; (2)Pa1-2, with one prealbumin band at the albumin and two bands at the anode end; (3) Pa2-1, with two prealbumin bands at the albumin and one band at the anode end;(4) Pa2-2,with two prealbumin bands at both albumin aad anode ends; and (5) Pa 0-1,with only one prealbumin band at the anode end.The frequency distribution of serum prealbumin phenotype is gradually concentrated on the phenotypes of Pal-1 and Pal-2 along with the increase of selective breeding degree. Whereas the original flock (group Ⅰ) has five phenotypes of serum prealbumin, its frequency distribution is dispersive. Pal-1 amounts to 35.42%, and the average egg-laying number per 300-day-old duck is 108.8 for this flock. After four generations of selective breeding, both the low-yielding family (group Ⅱ) and the high -yielding family (group Ⅲ)hare only three phenotypes. Their Pa 1-1 frequencies are 54.17 and 70.83%, coresponding to the average egg-laying numbers of 117.0 and 148.6 per 300-day-old duck and those of 233.88 and 271.56 per 500-day-old duck, respectively. Thus, a positive correlation between the egg-laying capacity and the frequency distribution of serum prealbumin phenotype of the Putian black duck has been established,that is, the average egg-laying number of the duck flock increases with the growing frequency of Pal-1.Therefore, the frequency distribution of serum prealbumin phentype provides a reliable index for early identifying fine families of duck.