We studied strains of an unusual streptococcus that superficially resembles Streptococcus sanguis but has fibrils that are arranged in lateral tufts. These strains were originally isolated from human throats and oral cavities and have been referred to previously as "Streptococcus sanguis I," the "CR group," and the "tufted-fibril group." Until now, insufficient phenotypic data have been available to allow reliable differentiation of these strains from other viridans streptococcal species, particularly the species in the S. sanguis group. Recently, workers have proposed a scheme of phenetic tests that is based on 4-methylumbelliferyl-linked substrates and conventional biochemical tests and allows the tufted-fibril group to be differentiated; these organisms differ from other viridans species in being able to hydrolyze arginine but not esculin and in producing alpha-L-fucosidase but not beta-glucosidase or alkaline phosphatase. These data, together with the results of our DNA-DNA hybridization experiments and the unusual ultrastructure of the tufted-fibril strains as determined by electron microscopy, demonstrate that these organisms represent a new species, for which the name Streptococcus crista is proposed. The DNA base composition is 42.6 to 43.2 mol% G + C. The type strain is strain CR311 (= NCTC 12479).