Genes of the Major Histocompatibility Complex (MHC) represent an essential component of the vertebrate acquired immune system. In the last decades, the role of MHC genes in mate choice has been subject of particular scientific interest. However, results of studies dealing with this topic in different species are equivocal and mechanisms conducting MHC-based mate choice are still puzzling. We investigated the impact of MHC class I variability on within-pair and extra-pair fertilisation success in a wild population of a socially monogamous passerine bird with considerable rates of extra-pair paternity, the scarlet rosefinch Carpodacus erythrinus. We found some support for the ‘good-genes-as-heterozygosity model’, as social males of high MHC-heterozygosity were cheated by their females less frequently than less MHC-heterozygous males. However, cuckolding males were not more MHC-heterozygous than the cheated social males, nor were extra-pair young more MHC-heterozygous than within-pair young. We did not find any evidence for mating preferences according to the complementarity model.