No structural change of the dopamine D2 receptor (DRD2) has been reported so far, though the DRD2 gene has been suggested to be one of the candidate genes for mental disorders. Herein we report one missense nucleotide mutation from C to G resulting in a substitution of serine with cystein at the codon 311 located in the third intracellular loop of the DRD2 that was found in the analyses of the sequence of the DRD2 gene in 50 schizophrenics. The allele frequency, 0.04, of this Cys311 allele in 50 schizophrenics was slightly increased compared with that, 0.023, in 110 controls though the difference was not significant. The schizophrenics with Cys311 tended to have a lower age of onset and a positive family history of schizophrenia.