Analyses of 165 rRNA gene sequences, restriction endonuclease digestion fingerprints of 16S-23S intergenic regions, DNA base compositions, fatty-acid profiles, cell-wall chemistry, cell physiology and fermentation end-product composition, along with other biochemical and phenotypic properties, supported the view that Trichococcus flocculiformis EchtT (DSM 2094T), Lactosphaera pasteurii KoTa2T (DSM 2381T), Ruminococcus palustris Z-7189T (DSM 9172T) and an isolate named 'Carnococcus allantoicus' NDP were all very similar and should be merged into a single genus. Detailed characterization of strains Ben 77, Ben 200 and Ben 201 described previously as 'Nostocoida limicola' I, a filamentous bacterium which causes bulking in activated sludge systems, revealed that these strains also belonged to the same genus as T. flocculiformis EchtT, L. pasteurii KoTa2T, R. palustris Z-7189T and 'C allantoicus' NDP. In fact, their shared properties suggested that these strains all belonged to a single species. However, DNA-DNA hybridization data indicated that T. flocculiformis EchtT, all of the 'N. limicola' I isolates and 'C allantoicus' NDP belonged to the same species, whereas L. pasteurii KoTa2T, R. palustris Z-7189T and two new isolates, 37AN3*T and 45AN2, represented three distinct species within the same genus. The priority of the genus name Trichococcus is established and since its validation predates the description of the genus Lactosphaera this name should take precedence. Under certain culture conditions, all of the strains mentioned above could produce chains of cocci. Furthermore, the morphology of T. flocculiformis EchtT could change to a non-filamentous form on certain media. This study proposes that the above strains be reclassified as members of the genus Trichococcus as four species, namely Trichococcus flocculiformis emend. (type strain EchtT = DSM 2094T), Trichococcus pasteurii comb. nov. (type strain KoTa2T = DSM 2381T = ATCC 35945T), Trichococcus collinsii sp. nov. (type strain 37AN3*T = DSM 14526T = ATCC BAA-296T, and Trichococcus palustris comb. nov. (type strain Z-7189T = DSM 9172T).