Cells of Rhv. sulfidophilum were grown under different conditions in the presence of P-32-phosphate and the corresponding H and L membrane fractions obtained and fractionated by SDS-PAGE, Both membranes showed almost identical polypeptide composition. The bacteriochlorophyll (Bchl) specific content in H was always lower that in L. As described before, oxygen did not regulate gene expression. Under high light, an almost two- to threefold decrease of the cellular specific Bchl content was observed. Pulse and chase experiments showed that transitions from aerobiosis to light-anaerobiosis did not quantitatively affect the Bchl content of the membranes, although a turnover of the P-32-phosphate and S-35-methionine was observed. LHI beta was the only polypeptidic subunit of the Bchl-binding polypeptides that was phosphorylated in vivo, and phosphotyrosine was the only phos phorylated amino acid detectable. The phosphorylated LHI beta was determined to be insoluble in the organic solvent mixture of(vol/vol) 1:1 chloroform-methanol containing ammonium acetate (0.1 M final concentration). Treatment with a chaotropic agent such as Na2CO3 solubilized the phosphorylated LHI beta, indicating that part of this posttranslationally modified polypeptide was not inserted in a transmembrane position. These results were used to speculate about the regulatory properties of this posttranslational modification of LHI beta on membrane differentiation.