The growth-related parameters of Thiobacillus intermedius, cultured in glutamate-CO2-S2O32- medium, have been determined. After centrifugation at 48,000 X g for 1 h, 24% of the D-ribulose-1,5-bisphosphate carboxylase (RuBPCase) activity of the disrupted-cell suspensions obtained from CO2-S2O32--and glutamate-CO2-S2O3(3)- grown cells could be sedimented, and the specific activities of this enzyme in the supernatant fractions were almost equivalent. The enzyme was stable in T. intermedius starved of thiosulfate in the presence and absence of glutamate, but a progressive decrease was evident in several growth cycles, each cycle supported by resupplementation of cells with thiosulfate. Polyhedral inclusion bodies were present in CO2-S2O3(2)- and glutamate-CO2S2O3(2)- grown cells. The number of polyhedral bodies per cell increased during mixotrophic growth approximately in proportion to the observed increase in the specific activity of RuBPCase. RuBPCase could not be detected in T. intermedius grown heterotrophically on yeast extract, nor could polyhedral bodies be found.