i. A four-year survey, involving the investigation of over 1,000 cases of strike, has been made of the species concerned in sheep myiasis throughout the British Isles. In all regions, L. sericata is the principal maggot-fly. In Scotland, Northern England and North Wales, L. caesar is an important species. Phormia terraenovae has much the same distribution, as a striking species, as L. caesar, except that it apparently does not strike sheep in North Wales ; in the areas where it occurs it follows L. caesar in importance. These two species are apparently capable of acting as primary flies. Calliphora erythrocephala, and, more rarely, Calliphora vomitoria occur fairly generally over the country, but only in very low incidence. Larvae of Muscina spp. have been recovered from only three cases, one each in Scotland, England and Ireland. The two closely related species, L. caesar and L. illustris have only been separated in a few instances, and there is not sufficient evidence for the latter species, but it would seem to be more common in lowland areas than in the north and west.

ii. The incidence of “ alternative ” species, i.e. species other than L. sericata, decreases from north-west to south-east of Britain ; they occurred in 47 per cent. of strikes north and west of a line from Inverness to Glasgow, and in over 20 per cent. of the cases between this and a line from East Lothian to Morecambe Bay, south of the Lake District. In the fringe of hill country east and south of this, e.g., the Cheviots, Pennines and Wales, about 10 per cent. of cases contained “ alternative ” species. Farther to the south-east and south “ alternatives ” were rare.

iii. Within the above belts of territory, the “ alternative ” species appear to have certain common regions which are favourable for the striking habit, although the limits of strike-distribution for each species are apparently not quite coincident. These “ favourable regions ” are discussed in the text. With Phormia terraenovae the habit appears to be restricted to favourable regions, where it occurs in moderately high incidence ; with L. caesar strike occurs in relatively high incidence in favourable regions, but is also present, in low proportion, throughout the country, while the Calliphora species strike in low proportion in favourable regions, and sporadically elsewhere.

iv. Phormia and L. caesar have a higher relative incidence in the first half of the fly season than in the later half ; C. vomitoria may be restricted to the extremes of the season-May, June and September.