Rhodobacter capsulatus was grown in chemostat cultures under different dilution rates and with ammonium ions as the limiting nutrient. The maximal growth rate (mu max) and the Monod cell growth saturation coefficient (K-s), were calculated from batch cultures grown at different concentrations of NH4+. The experiments in chemostat were carried out at 0.25 mM (NH4)(2)SO4, and the dilution rates were varied between 38% and 75% of mu max. The results indicated that under continuous culture conditions the cell yield coefficient (Y) (mg dry weight x mu mol consumed ammonium sulfate(-1)) decreased with increasing dilution rate (D). On the contrary, the cell yield was constant when expressed as mg cellular protein x mu mol consumed ammonium sulfate(-1). This occurred as a consequence of both an increase in the consumed ammonium sulfate and a simultaneous decrease in the cell biomass production at increasing growth rates. The cells produced at higher growth rates had a higher protein content per cell. The specific content of bacteriochlorophyll (Bchl) decreased (between 3 and 4 times) with increasing growth rates measured in either cells or chromatophores. However, the absorption spectra of the cells indicated that the ratio LHI (light-harvesting complex I) to LHII (light-harvesting complex II) Bchl complexes did not change. The reaction center (RC) complex content varied in parallel with the total Bchl content, yielding a constant photosynthetic unit of 65 mol Bchl x mol RC(-1) at different Ds. On the other hand, the uncoupled ATPase-specific activity measured in chromatophores was usually between 30% and 40% higher at the highest growth rates reached in these experiments.