We evaluated the role of the serotonin transporter gene promoter polymorphism (5-HTTLPR) in the etiology of dissociation. Adult twin pairs (N = 184 pairs; mean age 33.0 years, SD = 10.8) completed measures for dissociation and trauma. The DNA samples were genotyped for 5-HTTLPR adjusted for rs25531 alleles. Behavioral genetic analyses showed that genetic factors explained 45% of the variance in dissociative symptoms, while 55% of the variance was explained by unique environment and measurement error. Participants with the SS genotype of 5-HTTLPR reported more dissociative symptoms compared to participants with the other genotypes (p = .02), and they showed more pathological dissociative symptoms than the other participants (p = .04) when they reported more depressive symptoms and when they had experienced trauma.