The dopamine receptor D4 (DRD4) has received increasing research attention in behavioral science, psychiatry, and psychopharmacology. However, the number of available genetic markers for primates is still insufficient. We identified a novel variation/polymorphism in the second intron of DRD4 in humans based on the survey of 210 Japanese: a 6 bp insertion (allele frequency: 0.002) and 8 bp deletion (0.024); however, 94 Hungarian Caucasians were found to be monomorphic. Polymorphisms of the homologous region were also found in a survey of 93 specimens from four species of great apes and 51 specimens from seven species of gibbons. The polymorphisms consist of both single nucleotide substitutions and variations in the number of tandem duplications of short GC-rich sequences. Because of usefulness of primates in behavioral science, this polymorphism may be a useful marker for association studies with behavioral traits in both humans and apes. (C) 2004 Elsevier Inc. All rights reserved.