The reproductive cicle of two subtidal population of the sea urchin Tetrapygus niger was studied in Punta Lagunillas and Herradura Bay (IV Region) from July 1987 to September 1988. The cuali- and cuantitative analysis of the gonads showed that both populations exhibit an annual reproductive cycle. One month after total evacuation the gonads are mature again. This condition is maintained during most part of the year (nine months), pointing to a fast post-spawning gametogenesis and to an extensive reproductive period. Although there are seasonal variations in the gonad weight through the year, the histological study does not show a temporal sequence of the different phases of the reproductive cycle. T. niger reaches the peak of gonad maturity during winter, and spawning ocurrs at the end of this season or at the beginning of spring. The presense of mature gonads throughout the year together with significant loses, in the gonad weight exhibited in both populations, and the absence of a main reproductive season, could be interpreted as a consequence of partial evacuation of gametes, which was not detected in the historic-al analysis due to the fast dynamic of gametogenesis. This could explain the rapid population increase of T. niger and its present abundance in the intertidal and shallow subtidal of northern Chile.