Optimum cooling rate and cooling method for successful cryopreservation of sea urchin (Pseudocentrotus depressus) sperm were investigated by comparing survival and morphological changes of spermatozoa after freezing and the percentages of fertilized eggs after insemination of post-thawed sperm to fresh eggs. Optimum cooling rate from -10°C to -60°C was concluded to be 5-6°C/min, which could be achieved by methanol dri-ice method (IWATA et al. 1989) . The fertility of the sperm cryopreserved by the above method was estimated as 1% to that of fresh sperm.
