The acanthocephalan Echinorhynchus lageniformis Ekbaum, 1938, is a common intestinal parasite of the starry flounder, Platichthys stellatus (Pallas), of Yaquina Bay, Lincoln Co., Oregon. Three hundred and sixty-one flounders were examined and 146 (40.5 percent) were found to harbor infections of E. lageniformis. Cystacanths of E. lageniformis were found encysted in the body cavity of the starry flounder; this is probably the result of the fish feeding on larval infections not yet mature enough to attach to the intestinal mucosa. As the size of the flounder increased, the percent infection decreased. The smaller fish with the higher incidence of infection were found to be of the size range that ate amphipods, which may be the intermediate host, as the main part of their diet. The older fish do not feed on amphipods. A correlation could not be found between the sex of the starry flounder and the incidence and degree of parasitism. A peak of adult worms was found in April, 1966 - 198 mature females as compared to a low of 30 mature females in December, 1965. This indicates a seasonal periodicity in the E. lageniformis population of Yaquina Bay. Possible explanations for this seasonal occurrence were discussed. It is postulated that E. lageniformis lives in its final host for about a year. The one-year-old plus fish were found to have a high incidence of infection, whereas the two-year plus fish had a low incidence of infection. Twenty-six percent of the worms collected were males and 74 percent were females. Of the 146 infections, 74 were unisexual and 72 were mixed (containing both sexes). In April, 1966, the percent infection was the highest - when there were 20 mixed infections and two unisexual infections. This indicates that all the females have a good chance of being fertilized. The starry flounder intestine reacts to E. lageniformis at the point of attachment. It was observed that the larger the worm, the more severe the reaction. No crowding effects were observed in single species infections of E. lageniformis or in concurrent infections with trematodes. The shelled acanthor larva was found to have four enveloping membranes. Data indicates that when the proboscis reaches a certain length and width, it does not increase in size, although the neck and trunk of the worm do increase in size. / Graduation date: 1967