I. A description is given of 94 strains of urea-decomposing bacteria which may be placed in the B. pasteuri group; they were isolated from a variety of soils and from manures: (1) by enrichment cultures in media containing 5-10% urea, (2) by inoculating urea-containing liquid media with high dilutions of soils, and (3) by direct plating of soils on agar containing 5-10% urea. Method (1) permitted isolation of the most active urea-decomposing bacteria in a sample; methods (2) and (3) yielded cultures of the most numerous and, generally, of the least active organisms. The group consists of a series of strains among which there appear to be no natural divisions. Strains decomposing urea actively were almost incapable of developing in ordinary neutral media, but a few of the least active ones grew readily in these substrates. Individual strains exhibited variations in morphology of vegetative cells and spores, opt. temp. (ca. 25[degree]-37[degree]C), form of colony, and in action on gelatin and nitrates. A study of colony variants proved that differences between strains, in cell morphology and growth characteristics on solid media, are not constant, but their physiological properties are, in general, identical with those of the parent strains.[long dash]II. Most of the organisms required alkaline media containing NH3. For strains which bring about rapid decomposition of urea but are incapable of growth in ordinary neutral media, opt. growth conditions in bouillon are produced by adding ca. 1% NH4Cl and adjusting to ca. pH 9. Strains producing a weaker action on urea are less dependent on NH3 and alkaline reaction, but a few of them grow readily from small inocula on ordinary media. The most suitable substrates for the organisms contain urea from which the necessary NH3 is formed during sterilization, and later by the urease activity of the bacteria. Visible growth was not observed in synthetic sol. In media containing ordinary bouillon or a protein, the organisms, while incapable of growth in pure culture, may proliferate actively in presence of other bacteria. The appearance of nitrites in NH3-contaming cultures may be attributed to 1 or more of several circumstances, but not to an oxidation of NH3 by the organisms.