Abstract: This paper investigated erythrocytic antigen differences between individuals in R. C. Carp population using routine serlogical methods. This antigen system is nominated as S blood type (or group) system. Based on the results of isoagglutination and isoantibody and rabbit antiserum absorption tests, four blood phenotypes were revealed: no antigen(type Ⅰ, S0), two antigens(type Ⅱ, S1S2) and single antigen(type Ⅲ, S1 and type Ⅳ, S2). In serum, type Ⅰ has anti-S1 and anti-S2, type Ⅱ has no antibodies, type Ⅲ has anti-S2, and type Ⅳ has anti-S1. This blood type system is similar to the phenotype models of "A-B-O" type in tuna and ABO type in human being, but is different from the model in deer mouse which has only 3 phenotypes, A, AB and B, possibly controlled by two alleles alabelled PA and PB. It has been assumed that S blood type in R. C. Carp is determined by 3 multiple alleles labelled S1, S2 and S0 (here S1 and S2 are codominant, and S0 is most likely to be recessive). Although the genetic background of S system is possibly similar to that of human ABO. the results of agglutination test between human "A" and "B" serum and four types of S system in R.C.Carp showed that S system is a particular blood type independent of human ABO. The primary data present a much higher S1S2 ratio(6/16) in R.C.Carp than in human AB type. Further investigations of crossing between genotypes for S system are in process.