Although it is widely accepted that serotonin plays a pivotal role in the modulation of anxiety- and depression-related personality traits as well as in the pathogenesis of anxiety disorders and depression, the role of serotonin in cognition is less clear. In the present study, we investigated the involvement of serotonin in cognitive behaviors by examining the impact of genetic variation in key regulators of serotonergic neurotransmission on behavioral measures in a cognitive control task. Eighty-five healthy participants performed a cued continuous performance task (the AX Continuous Performance Task [AXCPT]) and were genotyped for polymorphisms in the transcriptional control regions of the tryptophan hydroxylase 2 gene (TPH2 G-703T; rs4570625) and the serotonin transporter gene (5-HTTLPR). The core result was that individuals lacking the rare TPH2 T allele were not faster than T allele carriers, but committed fewer errors and were less variable in responding. These findings parallel those of a recent study where an enhancement of executive control in individuals without the rare TPH2 T/T genotype was observed. Together with recent evidence that individuals without the T allele exhibit higher scores in anxiety- and depression-related personality traits, our results underscore the role of the TPH2 G-703T polymorphism in the modulation of behavior and raise the intriguing possibility that genetic variants associated with higher negative emotionality may have beneficial effects on some cognitive functions.