The population structure of the slate-pencil sea urchin Heterocentrotus mammillatus was studied along the southeastern coast of the Sinai peninsula. Evenly-distributed size classes were observed in populations occupying the forereef, while populations of this species from the reef slope and the back reef contained smaller proportions of small and medium-sized individuals. During the five years of study, growth rates and abundances of each size group were found to be stable. Analysis of size frequency distributions show that recruitment of H. mammillatus at the study area was regular and mortality rates were low during the second to fifth year of life and increasing later. This was supported by aging of dead individuals and observations indicating that large individuals were more susceptible to predation than smaller ones.