SummaryThe Pm blood group system inPeromyscus maniculatuswas originally described by Rasmussen as consisting of two co‐dominant alleles,PmAandPmB, responsible for the antigenic characteristics A and B. This paper describes two additional antisera, anti‐C and anti‐X, which expand this system and show it to be serologically complex. Anti‐X is comprised of antibodies directed against at least two antigenic characteristics X‘ and X“. Four and possibly six alleles are described by the four antisera. Different subspecies exhibit different spectra of alleles at thePmlocus. A segregation distortion i