One hundred and seventy-five isolates of the pathogenic bacterium Listeria monocytogenes recovered from human clinical (blood and cerebrospinal fluid), animal, and environmental sources in Europe, North America, and elsewhere were analyzed electrophoretically for allelic variation at 16 genetic loci encoding metabolic enzymes. Forty-five distinctive allele profiles (electrophoretic types, ETs) were distinguished, among which mean genetic diversity per locus (H) was 0.424. Cluster analysis of a matrix of genetic distances between paired ETs revealed two primary phylogenetic divisions of the species separated at a distance of 0.54. ETs in division I were presented by strains of serotypes 4b, 1/2b, and 4a, whereas strains of ETs in division II were of serotypes 1/2a and 1/2c.Human and animal isolates did not represent distinctive subsets of ETs. The occurrence of linkage disequilibrium between enzyme loci and the widespread distribution of certain ETs indicate that the genetic structure ofL. monocytogenes is clonal. One clone, marked by ET 1, caused major epidemics of human disease in western Switzerland in the period -1987 and in Los Angeles County, California, in 1985, both of which were attributed to contamination of soft cheese. ET 1 is closely related to the clone (ET 7) that caused two large outbreaks of listeriosis in Massachusetts in 1979 and Bacteria of the genus Listeria are widely distributed in the environment and also occur in the intestinal tract of healthy animals and humans (1). Among the several species of the genus, only Listeria monocytogenes is commonly pathogenic for humans (2, 3), in which it causes serious invasive disease (septicemia, meningitis, and meningoencephalitis), primarily in the immunologically compromised host, the neonate, and the fetus (1,2 3818The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.