Eggs of Pacific herring, Clupea harengus pallasi, collected from Prince William Sound, Alaska were exposed to Prudhoe Bay crude oil for 4–144 hours, then returned to uncontaminated seawater for further development. Newly hatched larvae from both control and experimental groups showed no gross abnormalities. Transmission electron microscropy revealed inter- and intracellular spaces in brain and muscle tissues of exposed organisms but not in those of controls. Many mitochondria in the body muscle of exposed organisms were swollen, some with deteriorating cristae structure.