Electrophoretic patterns on polyacrylamide gels of blood serum proteins from 74 mountain whitefish (Prosopium williamsoni Girard) were examined for differences as they pertained to two geographically isolated populations, age, and sex. Serum samples were acquired from two supposedly isolated populations, the lower Provo River and the lower Weber River. Comparisons among gels were made on the basis of protein distribution as indicated by number of fractions and their density. Serum proteins showed marked intraspecific differences between the males as well as the females of each population. No variations were found which could be attributed to degree of maturity. Definite pattern differences were observed between the sexes of each population. Individual pattern variations within each group studied were observed.