Using heated antigens, antisera were prepared against the somatic antigens of 16 strains of Serratia marcescens. By means of cross-agglutination tests it was possible to divide the strains into six groups. Groups II and VI each contained mutually adsorptive strains, which indicated that the members within respective groups are closely related. No correlation was observed of serological activity and pigment production among the pigmented and non-pigmented strains.