Mutants of Alcaligenes eutrophus were isolated on the basis of their inability to grow on succinate as the sole source of carbon and energy. The mutants also failed to grow on other gluconeogenic substrates, including pyruvate, acetate, and citrate. Simultaneously, they had lost their capability for autotrophic growth. The mutants grew, but slower than the wild type, on fructose or gluconate. Growth retardation on gluconate was more pronounced. The mutants lacked phosphoglycerate mutase activity, and spontaneous revertants of normal growth phenotype had regained the activity. The physiological characteristics of the mutants indicate the role of phosphoglycerate mutase in heterotrophic and autotrophic carbon metabolism of A. eutrophus. Although the enzyme is necessary for gluconeogenesis during heterotrophic growth on three- or four-carbon substrates, its glycolytic function is not essential for the catabolism of fructose or gluconate via the Entner-Doudoroff pathway. The enzyme is required during autotrophic growth as a catalyst in the biosynthetic route leading from glycerate 3-phosphate to pyruvate. It is suggested that the mutants accomplish the complete degradation of fructose and gluconate mutase lesion. The catabolically produced triose phosphates are converted to fructose 6-phosphate which is rechanneled into the Entner-Doudoroff pathway. This carbon recycling mechanism operates less effectively in mutant cells growing on gluconate.