Laboratory populations of the polychaete worm Neanthes arenaceodentata were exposed to acute doses of external gamma radiation to determine effects on reproduction. Groups of mated pairs received either no radiation (controls) or 0.5, 1.0, 2.0, 5.0 10 or 50 Gy. The doses were delivered at the time when oocytes were visible in the females and at a rate of 5 Gy/min. The broods from the mated pairs were sacrificed before hatching occurred, and information was obtained on brood size, on the number of normal and abnormal embryos, and the number of embryos that were living, dying, and dead. An important effect of acute irradiation was increased mortality of the embryos. Except for those mated pairs that received 10 or 50 Gy, there was no evidence for gamete loss or for reduced fertilization success; the number of embryos in the brood did not decrease with increased dosage. The results on embryo abnormality and mortality indicate that lethal mutations were most likely induced in the germ cells and that these affected survival of the early life stages.