Mismatched heteroduplexes arise as intermediates of several dissimilar genetic processes. The outcome of these genetic events will therefore be influenced by the efficiency and specificity of mismatch repair. We have studied the correction of base–base mispairs in simian and human fibroblasts by transfecting the cells with derivatives of SV40 DNA, each harboring a single mispair in a defined orientation. Analysis of plaques revealed that correction efficiencies for homomispairs followed the pattern G∙G > C∙C ≥ A∙A > T∙T. Repair bias was influenced by flanking sequences. Correction efficiencies for heteromispairs followed the pattern of G∙T > A∙C > C∙T > A∙G and repair favored the retention of G+C by a substantial margin. This repair specificity could lead to a gene conversion bias favoring the accumulation of G+C in sequences subject to high levels of recombination or unequal exchange.Key words: DNA repair, gene conversion, recombination, DNA methylation.