Polyacrylamide gel electrophoresis (PAGE) of sodium dodecyl sulphate (SDS) extracts of bacterial cells was used to assess the similarity of polypeptide patterns in 15 isolates of the rumen cellulolytic bacterium Bacteroides succinogenes. Analysis of densitometer traces of the polypeptide band patterns suggested that the majority of the isolates tested were closely related to the type strain S85. Representatives of other species of Gram-negative rumen bacteria had distinct SDS–PAGE patterns, and it is concluded that SDS–PAGE is a useful aid to the identification of B. succinogenes.