The dopamine transporter (DAT) is the site of action of stimulants, and variations in the human DAT gene (DAT1) have been associated with susceptibility to several psychiatric disorders including attention deficit hyperactivity disorder (ADHD) and bipolar disorder. We have previously reported the association of bipolar disorder to novel SNPs in the 3' end of DAT1. We now report the identification of 20 additional SNPs in DAT1 for a total of 63 variants. We also report evidence for association to bipolar disorder in a second independent sample of families. Eight newly identified SNPs and 14 previously identified SNPs were analyzed in two independent samples of 50 and 70 families each using the transmission disequilibrium test. Two of the eight new SNPs, one in intron 8 and one in intron 13, were found to be moderately associated with bipolar disorder, each in one of the two independent samples. Analysis of haplotypes comprised of all 22 SNPs in sliding windows of five adjacent SNPs revealed an association to the region near introns 7 and 8 in both samples (empirical P-values 0.002 and 0.001, respectively, for the same window). The haplotype block structure observed in the gene in our previous study was confirmed in this sample with greater resolution allowing for discrimination of a third haplotype block in the middle of the gene. Together, these data are consistent with the presence of multiple variants in DAT1 that convey susceptibility to bipolar disorder.