Summary and Conclusions.The occurrence, habitat, proportions of the sexes, and population distribution in different zones of the int(ertida1 region in Patella vulgata Linnaeus, on thoNorth Side of Cullercoats Bay, are described.Five main conclusions can be drawn from our investigations 011 the biology of Patella vulgata a t Cullercoats:1. Patella vulgata occurs mainly between the L.W.S. and tlie H.W.S. levels. It is, however, found in diminishing numbers right up to the wash zone (about 2 feet above the H.W.S.)on sides of rocks facing North‐East and East. The factor determining the upper limit of distribution is probably the absence of suitable food‐plants.2. In general the population increases from the high‐water to the low‐water mark; but the maximum population density occurs at low water neaps. At higher levels the larger sizes are predominant, while at lower levels it is the smller sizes which are most numerous. The largest sizes (46–60 mm.), however, are found more usually near the Gigartim belt of L.W.S. or on the margins of rock‐pools from L.W.S. t o M.T.L.3. The ratio of the males to fema1es varies inversely as the degree of exposure. The lower we go from the high‐water mark, the larger is the population of males. The larger males (i.e., persistent males) ‐are inore common in the higher zones, while the larger females are found more at lower zones of the coast.4. First Inaturity in P. vulgata occurs at a size of 10 mm. when all indi‐viduals with gonads are males; i.e., all P. vulqato are male at first matwity. The first females appear at sizes 12–15 mm., which we have considered the transition stage, and go on increasing in number gradually until the size‐group 21–25 min. is reached. Sbove this size there is asudden increase in tlic!percentage of females, which become more numerous until a t a size of 56‐40 mm they form about 8Oyh of the population. Correspondingly, there i s a suddendrop in the percentage of inales a t size 21‐25mm., above which size they become less numerous, until at a size of 56–60 mm. they form only 20% of the population. A 50; 50 proportion of, males to females is obtained only in the size‐group 3640 mm.5. The ratio Of males to females in the total popzllation is about 3: J., i.e., during September and October, 1946, at Cullercoats there were three functioning niale