The seasonal incidence curve of strike is bimodal, and divisible into four phases, the pre-shearing, the shearing trough, the lamb phase, and the late season adult and lamb phase. About two-thirds of the strikes reported by shepherds occurred in the hindquarters area (this term including the rump and hindlegs). In a lowland flock where all strikes were noted over a five-year period the incidence of hindquarters strikes was much lower (43 per cent.). The shoulders area, in relation to its size, is highly susceptible. It appears to be even more common a site in lowland than in mountain breeds. Rather more than half of the reported cases were in adult sheep, and of these more than half were in shorn sheep. Body strikes were relatively more common in shorn than in woolled sheep. Species other than L. sericata (referred to as alternative species) occurred mainly in mountain breeds or their lowland crosses, and more frequently in body than in breech strikes. Alternative species were more or less equally common in adults and in lambs, except Phormia terraenovae, which was practically confined to adults. The occurrence of alternative species was significantly associated with the type of grazing, and also with the vegetation type on the grazing ; the partial association with vegetation type, with pasture type constant, suggests that presence of bracken and/or heather is closely associated with the conditions resulting in alternative species striking sheep.