Three problems associated with the taxonomy and growth of anaerobic cocci belonging to the genera Peptococcus and Peptostreptococcus were studied. One study involved the designation of the neotype strain of Peptococcus niger, the type species of the genus Peptococcus. The type strain had been lost, and there were no extant ·strains of this taxonomically important species. Several strains of anaerobic cocci isolated from the navels of several human subjects were found to be identical with the original description of Peptococcus niger. Therefore, an amended description of this species was published. Another study involved the taxonomic status of Peptococcus anaerobius. A search of the early descriptions of Peptococcus anaerobius revealed that the original description of the species was probably based on descriptions of several different anaerobic cocci. Consequently, a request has been made of the Judicial Commission of the International Committee on Systematic Bacteriology to reject the name Peptococcus anaerobius as a nomen confusum. A third study involved the inhibition of growth of Peptostreptococcus anaerobius by sodium polyanethol sulfonate (SPS). Gelatin and protease peptone were found to protect Peptostreptococcus anaerobius from inhibition of growth by SPS, and, therefore, are useful additions to blood culture media for the purpose of enhancing isolation of Peptostreptococcus anaerobius from blood cultures. Numerous strains of other anaerobic cocci were also tested for sensitivity to SPS.