Marked Lucilia caesar group and Calliphora erythrocephala blowfiles released beside a 200-yard wide river were recaptured in numbers on the far side. In 2 late-summer and autumn tests of the barrier effect of a deciduous wood belt 90 yards wide, Lucilia sericata, L. caesar and Calliphora erythrocephala were recovered beyond wood; C. vomitoria was recovered beyond a 50-yard wide stand of conifers. Estimates are given of density in test areas of some of the species concerned. The bearing of the observations on dispersal theory is discussed.