We investigated the ability of Serratia marcescens to kill Manduca sexta (tobacco/tomato hornworm) larvae following injection of ca. 5 × 105 bacteria into the insect hemolymph. Fifteen bacterial strains were examined, including 12 non-pigmented clinical isolates from humans. They fell into 6 groups depending on the timing and rate at which they caused larval death. Relative insect toxicity was not correlated with pigmentation, colony morphology, biotype, motility, capsule formation, iron availability, surfactant production, swarming ability, antibiotic resistance, bacteriophage susceptibility, salt tolerance, nitrogen utilitization patterns, or the production of 4 exoenzymes: proteases, DNase, lipase, or phospholipase. There were marked differences in chitinase production, the types of homoserine lactone (HSL) quorum sensing molecules produced, and the blood agar hemolysis patterns observed. However, none of these differences correlated with the six insect larval virulence groups. Thus, the actual offensive or defensive virulence factors possessed by these strains remain unidentified. The availability of this set of S. marcescens strains, covering the full range from highly virulent to non-virulent, should permit future genomic comparisons to identify the precise mechanisms of larval toxicity.