The polychaet, Ophryotrodia puerilis, which is easily bred in the laboratory, lends itself well to research on the effects of radiation on invertebrates. Growth, which depends on the activity of a growth zone, is accomplished by the addition of segments. It is, therefore, easily recorded quantitatively. Radiation doses up to 100,000 r (3335 r/min) were used. The effect of the radiation depends on the size of animal. In animals of 20 segments the LD 50/30 dose is about 50,000 r; in animals of 10 segments doses of 20,000 r and greater completely inhibit growth, and doses of 10,000 r inhibit the growth of smaller animals. Animals with 10 segments slight retardation of growth after 5,000 and 10,000 r, while the smaller animals do not grow after 10,000 r of radiation and have their growth significantly retarded by 5,000 r. — Eggs laid by sexually mature females treated with 50,000 r fail to develop. After 20,000 and 10,000 r the eggs laid in the first 10 days do not develop but eggs exposed later are capable of develpoment. Animals of 10 segments are fertile after 10,000 r but sterile after 20,000 r. — Fractionation of the radiation dose decreases the mortality rate and, with a total dose of 20,000 r, allows growth. — Irradiation of eggs has an effect that is dependent on the age of the embryo. Up to 9 days 1000 r are lethal; from 10 days on 2000 r have no effect on development. Irradiation with 500 r shows no effect, while two 250 r increments of dose given 48 hours apart kill eggs 2 — 10 days old.