The following new combinations for names of species of phototrophic sulfur bacteria are proposed : (i) Thiocystis gelatinosa (Winogradsky) comb. nov., (ii) Thiodictyon bacillosum (Winogradsky) comb. nov., (iii) Amoebobacter pendens (Molisch) comb. nov., (iv) Pelodictyon luteolum (Schmidle) comb. nov. Each of the two species of Chlorobium Nadson 1906 has an infrasubspecific forma specialis designated: Chloro- bium limicola forma specialis thiosulfatophilum and Chlorobium vibrio- forme forma specialis thiosulfatophilum. A revised description of each newly named organism is given. Neotype strains are designated. The first comprehensive classification of the purple sulfur bacteria (Thiorhodaceae Molisch 1907, reference 9) was established by Wino- gradsky (19). It was based on careful micro- scopic observations of cell material which was collected in nature and kept for some time in crude cultures containing mud and water of the habitat. Differentiating criteria for the genera were the mode of development of cell aggre- gates, the structure of the aggregates, slime production, and the arrangement of the cells within the slime capsules (Bergey's Manual of Determinative Bacteriology, 7th ed.). During the past 10 years, most of the species described by Winogradsky have been isolated in pure culture and studied under varied experimental conditions (see 14). These studies revealed that the criteria used by Winogradsky are highly dependent on the growth conditions. Conse- quently, a classification of the Thiorhodaceae based on pure-culture studies has to rely primarily on the characteristics of the single cells. If the morphology of the cells is con- sidered in the first place, most of Winogradsky's original genera and species can be maintained, though certain nomenclatural rearrangements become necessary. Therefore, we propose the following rearrangements, which are in agree- ment with the International Code of Nomencla-