The ts8 mutant of Escherichia coli has previously been shown to preferentially inhibit stable RNA synthesis when shifted to the nonpermissive temperature. We demonstrate in this report that the ts8 mutation is an allele of fda, the gene that encodes the glycolytic enzyme fructose-1,6-diphosphate aldolase. We show that ts8 and a second fda mutation, h8, isolated and characterized by A. Böck and F. C. Neidhardt, are dominant mutations and that they encode a thermolabile aldolase activity.