Anxiety is a normal aspect of human personality, which can manifest in a variety of disorders and other negative traits. The primary treatment for anxiety is the class of drugs known as the selective serotonin reuptake inhibitors (SSRIs), which bind to the serotonin reuptake transporter. The upstream region of the gene that codes for this transporter contains a polymorphism that is an insertion/deletion event that in turn, produces long (l) and short (s) alleles in the population. This particular polymorphism in the serotonin transporter, the 5HTTLPR (serotonin transporter linked polymorphic region), is thought to be involved in the genesis of anxious traits and disorders. Most studies with human subjects have examined adult behavior, which may derive from diverse experiential and environmental backgrounds, as well as genetic differences. To better isolate the effect of genetics, we genotyped 128 infant and juvenile monkeys for the 5HTTLPR and tested for behavioral response in four testing paradigms designed to elicit fearful-anxious behaviors: a free play, remote-controlled car, human intruder, and novel fruit test. The s/s monkeys were found to be behaviorally inhibited in the free play test, engaged in more fear behaviors in the remote-controlled car test, and threatened more in the stare portion of the human intruder test, even though a small number of monkeys were assessed. There was no difference between genotypes of either sex in the prolactin response to fenfluramine. These data indicate greater anxiety in the s/s monkeys for distinct facets of anxious behavior, which are independent of a global neurohormonal challenge test. These neurobehavioral data support recent neuroimaging findings in humans indicating the importance of the 5HTTLPR for amygdala-dependent anxious behavior.