A collection of 159 strains of “black yeasts” obtained from a number of culture collections and a wide variety of substrata was studied. Morphologically, four species were present:Phialophora jeanselmei with a mycelium that produced radulaspores and also phialides;Sclerophoma pithyophila, which produces pycnidial initials and sometimes pycnidia in culture; and two species ofAureobasidium, A. pullulans with wide mycelium and large radulaspores, andA. mansonii, which produces narrow mycelium and small radulaspores. Physiologically, these species form two distinct groups:A. mansonii andP. jeanselmei, which grow slowly; andA. pullulans andS. pithyophila, which grow more rapidly. Except for rate of growth, there was little difference between these species in their abilities to use the carbon and nitrogen sources tested. None produced significant capabilities for fermentation in the sugars and at the pH level tested. All varied in their response to a technique designed to measure amounts of growth by turbidometry. The “black yeasts”, especiallyA. pullulans, are a highly polymorphic group of organisms capable of growing in a wide variety of locations. For the most part they are capable of using the simpler organic compounds found in nature, which indicates they apparently act largely on terminal or subterminal products of decay in the final stages of the mineralization of organic compounds.