From 1969 to the present the authors have maintained a regular log of the opistho­ branchs seen on Guam. Most of the study has been done in the area of Bile Bay, near the village of Merizo. During this time over a thousand specimens of Gastropteri­ dae, representing seven species, have been observed. Only a few of each species were collected, external descriptions were made, and color photograph slides taken. Until recently no effort was made by the authors to describe these animals. Gastropteridae are cephalaspid animals with paired parapodia which extend up over the dorsum, an elongate tapered tail, a posterior funnel on the cephalic shield, and a relatively short free mantle margin on the right side with the ctenidium usually located just below the mantle margin. In 1964 Tokioka and Baba divided the Gastro­ pteridae into two genera : Gastropteron characterized by a radular formula of 4-6. 1. 0. 1. 4-6 and with denticulations on the inner edge of the main hook of the first lateral; Sagaminopteron characterized by a radular formula of 9-12. 1. 0. 1. 9-12, with two promi­ nent cusps on the inner edge of the main hook of the first lateral. Carlson and Hoff (1973) described two new species of Sagaminopteron which had fewer marginals than the radular formula given by Tokioka and Baba, but the marginals were still more numerous than in Gastropteron. In 1967 a third genus, Enotepteron, was established by Minichev for a species collected near Tsingtao in the Yellow Sea. Enotepteron has four prominent cusps on the inner edge of the first lateral and a radular formula of 3. 1, 0. 1. 3. Only two of the genera, Gastropteron and Sagaminopteron, have been found on Guam. All external descriptions and comparative measurements presented here are from living animals. Since Gastropteridae change shape as they move, all such comparisons should be considered relative. Specimens of new species described in this paper are deposited in the Bernice P.