The purple, nonsulfur phototrophic bacteriumRhodobacter capsulatushas recently been found to contain several pigment biosynthetic operons that exhibit marked transcription read through into downstream operons that encode polypeptide components of photosynthetic pigment-protein complexes. This phenomenon has been found to be phenotypically significant for adaptation to changes in environmental conditions, and the term superoperon has been proposed to describe this sort of coupled transcriptional arrangement. We summarize the data that led to recognition of this novel transcriptional arrangement and suggest that superoperons might be more prevalent in prokaryotes than has heretofore been recognized.Key words: superoperons, overlapping transcription units, photosynthesis genes.