The effects of delayed spawning by adult Pacific herring Clupea harengus pallasi on the viability of eggs and larvae were examined. Adults were captured 1–2 months prior to spawning and held in captivity. They completed maturation (ovulation), did not spawn, and maintained sexual readiness for 2–3 months beyond normal spawning time. During this period, eggs were fertilized artificially and incubated. At regular intervals fertilization rate, survival to hatching, hatching rate, and viable hatch were monitored. Spawning delays of less than 2 weeks after maturation had no detectable effect on fertilization, survival, or hatching rates. Longer delays resulted in progressive loss of egg and larva viability, although fertilization rates remained high, and time to hatching decreased as the holding period increased. When females were held at colder temperatures, deterioration in the rate of egg and larva survival was delayed. If mature females are held longer than 2–3 weeks prior to experimental work on eggs and larvae, abnormally low estimates of progeny survival could result.