Wave action was responsible for movement of L. obtusata and caused high mortality on some shores by washing the animals on to the strand line. L. obtusata from wave exposed shores were adapted to wave action by crevice seeking behaviour, laying eggs on rock rather than weed, the young taking refuge within bladders of F. vesiculonust and the adults being able to maintain their position in strong water currentag partly by possessing a more streamlined shell. The stronger the wave action, the smaller the adult, due partly to lowered winter growth on exposed shores. Conditions of extreme shelter were better tolerated by animals inhabiting extremely sheltered shores than by those inhabiting the more exposed shores. Animals from extremely sheltered shores were less active in summer than in winter,and less active than animals from the more exposed shores. They were also better able to withstand high temperatures and lowered oxygen tensions. Carcinus maenas caused considerable mortality at Ynys Faelog, and attacked an introduced population till the density fell to approximately that of the surrounding indiginous population. Low down on certain shores were found animals with crab resistant shells, though the reason for the resistance was obscure. L. obtusata exhibited a variable geotaxis when out of water in the field, but a constantly negative geotaxis out of water in the laboratory, the rate of movement being slower when Fucus was the substrate provided. Geotaxis was always positive under water. The behaviour is explained as an adaptation to regaining their position on Fucus if dislodged. A marked positive rheotaxis was observed in a current of deep water, but not demonstrated in a film of moving water. L. obtusata maintained their position under Fucus as they were negatively phototactic. Animals on more exposed and on densely populated shores were smaller and laid fewer eggel though the individual eggs were of the same size. Natural populations were seen to exhibit a bimodal frequency distribution. Young were mainly hatched in the summerp mature the next summer and died the spring and following summerg breeding continuously after sexual maturity. No active migration of different size grouPs was in evidence. primary production of Fucus and its rate of consumption by L. obtusata. on a densely populated shore were estimated. Food may have limited the size attained at Trearddur Bay. The distribution of the following digenean Trematode parasitee on Anglesey were noted; Cryptocotyle lingua (Creplin), Hicrophallus simili (Jagerskiol), and Cercaria lebouri (Stunkard). There was a seasonality in the infestation rate. Females and sales were equally paraBitised. The sex ratio was biased slightly in favour of females. The parasites caused an increase in size attained by the hostq and mixed infections were rarer than expected.