Observations were made in Thetford Chase, East Anglia, from 1952 to 1957. Larvae spend winter and pupate in chambers just beneath surface of cones of scots pines (Pinus sylvestris), and less often, of Corsican pines (P. nigra). Blue tits (Parus caeruleus) and coal tits (P. ater) prey upon larvae in winter; while red squirrels (Sciurus vulgaris) destroy many cones in which larvae live, to eat seed. Larvae are parasitized by ichneumonid Ephialtes laticeps Ratz. Tits ate about 45% of populations of larvae each winter. In one winter with a very heavy crop of cones, tits'' predation was strikingly dependent upon density of larvae, reaching 70% where this was high. In another winter with a light crop of cones, tits'' predation was hardly density-dependent. In a forest compartment of 6.5 ha, tits'' predation was still influenced by differences in intensity of larvae from one part of this compartment to another. In general, more larvae per cone, greater proportion eaten by tits. Tits ate a greater proportion of larvae on outer quarter of cone than those on hidden quarter of cone. A greater proportion of larvae on inner than on outer quarter of cone is parasitized, but fewer than about 5% of all larvae were parasitized in each of 2 years. It is not known if tits select for or against parasitized larvae. Efficiency with which tits exploited those places and situations in which they could find larvae most readily was a significant feature of their predation; density-dependence of their predation is explained on these grounds. In 3 years initial crops of scots pine cones ranged from 7 to 39 cones/m2 of forest, of which squirrels ate from 16% to 65%. Initial crops of Corsican pine cones were 0.2 and 0.6 cones/m2 in 2 years, of which squirrels ate 80% and 93%. It is concluded that tits may normally be density-dependent predators, and squirrels density-independent predators, on larvae. Destruction of pine seed by larvae and squirrels is economically unimportant in Thetford Chase.