Holothuria atra is the most common aspidochirotid holothurian on tropical Indo-Pacific reef-flats. Transverse fission followed by regeneration has been studied in populations from New-Caledonia and Papua New Guinea. Fission has been observed in the field in all sizes of individuals at a mean rate of 1 % of the population sampled in New-Caledonia. From the direct observations and from the sizes of recently divided individuals, regenerating the oral or anal end, the position of the split has been located in the anterior 45 %. From the observed fission and regeneration rates in the population, it is infered that external regeneration (disappearance of fission signs) takes about two months. Asexual and sexual reproduction appears to be seasonal. But whereas fission mainly occurs during the cool season, sexual reproduction (from gonad studies) takes place during the warm season. Fission is probably triggered by emersion, during low tides, trhough dessication and thermal stress. It is hypothesized that the low water time of the spring tides can explain the seasonality of the fission as this phase of the tide occurs in these localities near the middle of the day in the cool season (and during the night in the warm season).