Two hundred and twenty-eight cultures representing the genus Listeria and a number of possibly related taxa were studied. Two hundred and two cultures (representing 187 strains and 15 duplicate cultures) were tested for 140 characters based on their morphology, physiology and biochemistry. The results were subjected to computer analyses. Two of the cultures were discarded due to contamination. Thus the total number examined was 200. On the basis of groupings so obtained, representative strains from the numerical taxonomic study and some others were examined by serological and, or, chemical techniques. The serological technique used was agglutination. The chemical techniques included cell wall and lipid analyses, cytochrome investigations and estimation of % G+C base ratios. The results indicate that the genus Listeria comprises only two species, L. monocytogenes (including L. monocytogenes. "L. innocua" and "L. bulgarica") and L. gravi (including L. gravi and L. murrayi). Listeria denitrificans should be removed from the genus Listeria. The chemical studies in particular indicate a relationship between the genera Listeria and Brochothrix at the suprageneric level. The name Listeriaceae is proposed for this family. The atypical lactobacilli Group 1 of Thornley & Sharpe (1959) should be designated thermosphacta. The bacteria of Groups 2 and 3 of Thornley & Sharpe (1959) are distinct taxa but their exact taxonomic placement was not resolved in the present study. The cultures labelled 'atypical Erysipelothrix' do not belong to the genus Erysipelothrix. Listeria is quite distinct from the genera Lactobacillus. Erysipelothrix, Curtobacterius and Kurthia.