The photosynthetic bacterium Rohodospirillum rubrum evidenced tyrosine protein phosphorylation under photoautotrophic conditions in the presence of [32P]Pi. The stability to alkaline treatment of the [32P] bound to the cell-free extract proteins suggested that tyrosine residues were carrying the labelling. One- and two-dimensional high voltage paper electrophoresis analysis revealed that such extracts do contain [32P]-phosphotyrosine residues. Furthermore, the association of alkali stable [32P] bound to specific proteins of the cell-free extract was confirmed by sodium dodecyl sulfate polyacrylamide gel electrophoresis combined with KOH treatment of the gel. A definite argument in favor of protein kinase(s) phosphorylating tyrosine residues in R.rubrum proteins was obtained by partial purification of a tyrosine kinase activity from cell-free extract capable of phosphorylating synthetic peptides that only contain a single tyrosine residue as phosphate acceptor.