In sugar beet, cytoplasmic male sterility (CMS) is conferred by the Owen mitochondrion (Svulg). In order to find polypeptides specific to this cytoplasm and putatively involved in CMS, we assessed the protein expressions of Svulg and a non-sterilizing mitochondrion (Nvulg) by in organello protein synthesis of mitochon-dria isolated from leaves. Given the hydrophobicity of mitochondrial translation products, we compared the in organello synthesis polypeptides of both cytoplasms with an acid-base two-dimensional electrophoresis adapted to hydrophobic protein separation. To evaluate the possible effect of nuclear background, we assessed the mitochondrial protein expression in three different nuclear backgrounds by using three near-isogenic-line pairs. While three to four variant polypeptides were revealed for each nuclear context, each variant polypeptide was specific to a nuclear-cytoplasmic context. Although this study did not enable us to unambiguously find any variant polypeptide related to CMS, we did observe an effect of the nucleus on mitochondrial gene expression.