The ecology of reproduction of the irregular lamp urchinEchinolampas crassa, including the cycle of reproduction, sex ratio, and the feeding and burrowing habits, is reported. Gametogenesis takes place during rising sea temperature and spawning during falling temperature. Reproduction is well synchronized within and between sexes and the greatest densities coincide with gametogenesis, the lowest densities with spawning. Discriminatory behaviour maximizes feeding efficiency and concentrates animals on ripple slopes.