Many symbiotic associations involve microorganisms which cannot be cultivated on laboratory media. These organisms remained little known until the recent advent of methods of recombinant DNA analysis and molecular phylogenetics. Applications of these methods to endosymbionts have resulted in substantial new insights concerning the genetics and evolution of these organisms. This communication provides a listing of recently studied associations involving non-cultivable symbionts. The associations involve a diverse set of host taxa and a wide range of effects, both favorable and deleterious, on host biology. Among beneficial endosymbionts, a variety of nutritional interactions have been documented. One type of association has been demonstrated for a number of animal hosts, namely endosymbioses that result from a single infection of an ancestral host by a prokaryote. In these associations, endosymbionts are transmitted maternally and are not exchanged between host lineages, resulting in a long-term pattern of codiversification of hosts and endosymbionts. The association between aphids and non-cultivable prokaryotic endosymbionts is a well studied example of such a symbiosis.