A conventional two-step freezing procedure was developed and optimized in order to cryopreserve oyster embryos. The effects of cooling rate, choice of cryoprotectant, and seeding temperature on the survival of late-stage oyster embryos were examined. When these factors were optimized, improved survival rates of 78 +/- 8 and 83 +/- 7% were achieved using 2 M Me(2)SO or glycerol, respectively, as the cryoprotectant. The experimental results indicate that oyster embryos survive after freezing over a broad range of cooling rates ranging from -0.5 to -16 degrees C/min. Me(2)SO, glycerol, propylene glycol, and ethylene glycol may be used as cryoprotectants for the cryopreservation of oyster embryos. Copyright 1999 Academic Press.