SUMMARY A numerical study of 60 strains of Serratia showed three phena. Phenon A included 5 I strains corresponding to the usual description of Serratia marcescens. Phenon B included five strains characterized by their fermentation pattern, positive malonate test and halotolerance. Phenon C included three aerogenic, cold-tolerant strains resembling pattern 2 of Fulton, Forney & Leifson (1959). One strain did not fall into any of these phena. All strains of human origin fell into phenon A. This phenon could be subdivided into two smaller phena, one composed of haemo- lytic and multi-resistant nosocomial strains. A hypothetical median organism for phenon A is described.