The following four new species of anaerobic gram-negative bacilli isolated from the human gingival crevice are described: Oribaculum catoniae, with ATCC 51270 as the type strain; Catonella morbi, with ATCC 51271 as the type strain; Hallella seregens, with ATCC 51272 as the type strain; and Johnsonella ignava, with ATCC 51276 as the type strain. C. morbi is associated with periodontitis. H. seregens and J. ignava are associated with gingivitis and periodontitis. O. catoniae has been isolated from healthy and diseased gingiva. Dialister pneumosintes (Olitsky and Gates 1921) gen. nov., comb. nov., nom. rev., associated with gingivitis, is proposed to accommodate organisms formerly classified as Bacteroides pneumosintes.