Holothuria atra is the most common aspidochirotid holothurian on tropical Indo-Pacific reefs, Asexual reproduction by transverse fission, followed by regeneration, has been studied at Reunion Island (Indian Ocean) and compared with different populations of the Indo-Pacific zone, thus permitting a better identification of the most significant parameters and a better understanding of this reproductive strategy. At Reunion Island, the species is studied at two stations on the same fringing reef: 1) on the back-reef where the fission rate is high (20 % of the population), the individuals small (generally weighting less than 150 g) and the population density high (4/m(2)); and 2) on the reef front, where fission is extremely rare, the mean size of the individuals larger (up to 300 g) and the density low (0.01/m(2)), Different categories of individuals, fissioning (F), after fission, anterior and posterior parts (A and P), and regenerating (Ap and Pa) have been identified from external observations. Dissection has demonstrated the unequal allocation of organs during fission and the variability of the regenerative states, mostly in the anterior part. Concerning fission, the position of the split in an individual is in the anterior half (at 44 % of the total length). The monthly incidence of fission is higher from October to January and in June-July. Fission does not result in an increase in the density of the population. The monthly rates of regenerating individuals originating from anterior (3.7 %) and posterior (5.1 %) parts suggest that survival is higher in the latter instance. The occurrence of asexual reproduction in various populations of this species is discussed in relation to the ecology and the parameters of the populations considered. On the back-reef studied at Reunion Island, the population is subtidal and emersion cannot explain fission. Anthropogenic disturbances are possible triggers of this phenomenon.