Pseudomonas oxalaticus was grown in carbon-and energy-limited continuous cultures either with oxalate or formate or with mixtures of these substrates. During growth on the mixtures, simultaneous utilization of the two substrates occurred at all dilution rates tested. Under these conditions oxalate repressed the synthesis of ribulosebisphosphate carboxylase. The degree of this repression was dependent on the dilution rate and the ratio of oxalate and formate in the medium reservoir. At a fixed oxalate/formate ratio repression was greatest at intermediate dilution rates, whereas derepression occurred at both low and high dilution rates. Progressive derepression of ribulosebisphosphate carboxylase synthesis and of autotrophic CO2 fixation at low dilution rates was attributed to the decreasing concentration of intracellular repressor molecule(s), parallel to the decreasing concentration of the growth-limiting substrates in the culture. To account for the derepression at higher dilution rates, it is proposed that the rate of oxalyl-CoA production from oxalate limits the supply of metabolic intermediates and that additional energy and reducing power generated from formate drains the pools of metabolic intermediates sufficiently to lower the intracellular concentration of the repressor(s). During growth of Pseudomonas oxalaticus on the "heterotrophic" substrate oxalate alone, at dilution rates below 10 % of the maximum specific growth rate, derepression of ribulosebisphosphate carboxylase synthesis and of autotrophic CO2 fixation was observed to a level which was 50 % of that observed during growth on formate alone at the same dilution rate. It is concluded that in Pseudomonas oxalaticus the synthesis of enzymes involved in autotrophic CO2 fixation via the Calvin cycle * To whom offprint requests should be sent Abbreviations. RuBPCase = ribulosebisphosphate carboxylase; PMS = phenazine methosulphate; DCPIP = 2,6-dichlorophenolindophenol; F D H = formate dehydrogenase; S R = concentration of growth-limiting substrate in reservoir is regulated by a repression/derepression mechanism and that the contribution of autotrophic CO2 fixation to the biosynthesis of cell material in this organism is mainly controlled via the synthesis of these enzymes.