AbstractMale sterileBrassica napusL. plants were found in breeding material which was used for the development of yellow‐seeded oilseed rape. The genetic studies indicated that the male sterility was conditioned by the presence of maintamer genes in the nuclear backgrounds of two newly resynthesizedB. napuslines, No7076 and No7406, in combination with a male sterility‐inducing cytoplasm (S) which is frequently found in cultivated forms ofB. napus.Test crosses withnapmaintainer and restorer lines support the conclusion that the observed male sterility is ofnaptype. Furthermore, the Eco RI restriction pattern of mitochondrial DNA of the (S) cytoplasm was identical to that of thenapcytoplasm. Hence, we conclude that we have uncovered a new source of maintainer lines for thenapsystem which couldpotentiallylead to the production of a better maintainer/restorer system for use in hybrid oilseed rape breeding programmes. However, more work is needed to reduce the glucosinolate content of the maintainer lines and to determine the factors controlling the phenotypic expression of the sys