1. Several nuclear mutants have been isolated which showed thermo- or cryo-sensitive growth on non-fermentable media. Although the original strain carried mitochondrial drug resistance mutations (CR, ER, OR and PR), the resistance to one or several drugs was suppressed in these mutants. Two of them showed a much reduced amount of the mitochondrial small ribosomal subunit (37S) and of the corresponding 16S ribosomal RNA. Two dimensional electrophoretic analysis did not reveal any change in the position of any of the mitochondrial ribosomal proteins. However one of the mitochondrial ribosomal proteins. However one of the mutants showed a striking decrease in the amounts of three ribosomal proteins S3, S4 and S15. 2. Four temperature-sensitive mitochondrial mutations have been localized in the region of the gene coding for the large mitochondrial ribosomal RNA (23S). These mutants all showed a marked anomaly in the mitochondrial large ribosomal subunit (50S) and/or the corresponding 23S ribosomal RNA.