Although mice with a targeted disruption of the serotonin transporter (SERT) have been studied extensively using various tests, their complex behavioral phenotype is not yet fully understood. Here we assess in detail the behavior of adult female SERT wild type (+/+), heterozygous (+/−) and knockout (−/−) mice on an isogenic C57BL/6J background subjected to a battery of behavioral paradigms. Overall, there were no differences in the ability to find food or a novel object, nest‐building, self‐grooming and its sequencing, and horizontal rod balancing, indicating unimpaired sensory functions, motor co‐ordination and behavioral sequencing. In contrast, there were striking reductions in exploration and activity in novelty‐based tests (novel object, sticky label and open field tests), accompanied by pronounced thigmotaxis, suggesting that combined hypolocomotion and anxiety (rather than purely anxiety) influence the SERT −/− behavioral phenotype. Social interaction behaviors were also markedly reduced. In addition, SERT −/− mice tended to move close to the ground, frequently displayed spontaneous Straub tail, tics, tremor and backward gait – a phenotype generally consistent with ‘serotonin syndrome’‐like behavior. In line with replicated evidence of much enhanced serotonin availability in SERT −/− mice, this serotonin syndrome‐like state may represent a third factor contributing to their behavioral profile. An understanding of the emerging complexity of SERT −/− mouse behavior is crucial for a detailed dissection of their phenotype and for developing further neurobehavioral models using these mice.