Developmental changes in the swimming behavior and underlying motoneuron activities of the larval angelfish, Pterophyllum scalare, were investigated in the course of post-embryonic development of the fish. For a few days after hatching, angelfish larvae showed spontaneous wriggling movement, which was alternating lateral bending of the tail without locomotion. On the 5th or 6th day after hatching, the larvae started swimming (locomotive activity). Spontaneous ventral root activity was recorded extracellularly from the body surface of the immobilized larvae as a monitor of the spinal motoneuron activity underlying wriggling and swimming. The number of motoneuron impulses and the duration of the burst in each cycle during the spontaneous rhythmic ventral root activity were increased as the larvae developed. In addition, the duration of the motoneuron bursts in older larvae was much more variable compared with that in early larvae, suggesting that the force of tail beat could be regulated flexibly as the larvae developed. In early larvae, two distinguishable patterns of spontaneous ventral root activity were observed: a low frequency bursting and a high frequency bursting. Such bursting patterns were consistent with the behavioral observations of free moving larvae. After the start of swimming, the frequency of the motoneuron bursting converged into a narrow range, corresponding to the stable, continuous swimming rhythm of the free moving larvae. We propose that the early stages of the angelfish larvae provide us with a useful model for investigating the developmental changes in neuronal substrates underlying swimming activity of the teleost fish.