An organism isolated from soil, cultivated in a simple medium containing only inorganic salts and sugar, was found to synthesize an essential growth factor that was nutritionally equivalent to soil extract in promoting the growth of an organism for which the latter was hitherto considered essential. The factor could not be replaced by the B vitamins, purines, pyrimidines, or other growth-promoting substances tested (including derivatives of various natural products) with the exception of hemin, which evoked but a partial response, and concentrated liver extract, the effect of which was not attributable to vitamin B12. Partially purified material from the metabolic liquid showed distinct growth-promoting activity at a concentration of 0.1 p.p.m. The organism requiring the growth factor, as well as that synthesizing it, both pleomorphic bacteria of the "soil diphtheroid" type, were found to be hitherto undescribed species. They are considered to be most appropriately assigned to the genusArthrobacter, as proposed by Conn and Dimmick, and are named, respectively,Arthrobacter terregensn. sp. andArthrobacter pascensn. sp.