5HT modulates the rhythmic locomotor output of most vertebrates by enhancing the duration and intensity of motor bursts in each cycle, but there is little clear evidence on the pharmacological profile of the 5HT receptor subtype(s) involved. In this study we extend our previous work on the role of 5HT in the development and modulation of locomotor behaviour in newly hatched Xenopus tadpoles by examining the 5HT receptor type responsible for enhancing the swimming activity in immobilized preparations. By applying a range of agonists and antagonists against different 5HT receptor subtypes, we conclude that serotonergic modulation of swimming activity is accomplished via the activation of just one receptor type with a pharmacological profile similar to the mammalian 5HT1a receptor. The effects of 5HT on burst duration (an increase) and on episode length (a decrease) are mimicked by the 5HT1a receptor agonists, 5-carboxamidotryptamine (5CT) and R(+)-8-OH-DPAT, and reversed by the 5HT1a receptor antagonist NAN-190. Agents acting at other 5HT1, as well as 5HT2 and 5HT3, receptor subtypes were without noticeable effect on the 5HT-enhanced swimming rhythm.