Spirochaetes within the Leptospiraceae family, encompassing the genera Leptospira, Leptonema and Turneria, are known to display substantial phenotypic and genetic variation resulting in their capacity to colonize diverse natural habitats worldwide. There are many lines of evidence to suggest that these microorganisms belong to at least three ecotypes. The first type consists of free-living nonparasitic leptospires inhabiting water, including marine and soil environments (Leptospira biflexa sensu lato). Not infrequently, saprophytic leptospires contaminate nutrition media, their occasional alleged isolation from mammalian hosts or humans resulting not only in misdiagnosis, but also in taxonomic confusion. The second ecological type consists ofparasitic leptospires showing pathogenic potential for humans and various animal species as well as host and tissue specificity of a wide spectrum (L. interrogans sensu lato). The third group comprises leptospires with life styles intermediate between independent and symbiotic (also within L. interrogans sensu lato). The latter (for example, L. kirshneri serovar grippotyphosa) display features of so-called sapronotic agents: high survival and competitive potential in environmental ecosystems, widespread mammalian host range and pronounced antigenic and genetic heterogeneity. This paper addresses ecological and genetic diversity within the Leptospiraceae family as relevant to epidemiological and taxonomic issues.