S U M M A R YThe enzymes involved in the tricarboxylic acid cycle and some other related reactions were measured quantitatively in cell-free extracts from the obligate autotroph Thiobacillus denitrijicans grown anaerobically, and in the facultative autotroph Thiobacillus-A2 grown under various conditions. The presence of an incomplete tricarboxylic acid cycle in T. denitrijicans and in Thiobacillus-A 2 (when grown autotrophically on thiosulphate) is described. The cell-free extracts from these two organisms lacked a-ketoglutarate dehydrogenase and succinyl CoA synthetase. The data from Thiobacillus-A z grown on succinate, aerobically or anaerobically, and on glutamate aerobically suggest that both the enzymes are repressed during autotrophic growth. Results are further discussed in relation to possible reasons for autotrophy.