The previously described acatalasic rho+ strain R-6 has been studied in order to determine the type of mutation responsible for its inability to produce catalase. Induction conditions for catalase activity and temperature influence on the behaviour of this strain were assayed. Furthermore, haploid progeny arising from crosses between R-6 and other strains (rho+, rho- and rho0) was examined for catalase levels. From the data obtained a hypothetical interaction between nuclear and mitochondrial genomes in relation to catalase biosynthesis is discussed.