The factors which influence the passage of flying and crawling insects through a doorway were investigated. It was considered practicable to exclude flying insects from a building or to prevent their successful escape from cages or rearing rooms by means of a downward air current if the speed of this in ft./min. multiplied by the ratio of the length through the doorway to its height is not less than 550. A combination of reflex and illuminated edges, smooth shiny walls without a pattern visible to insects, and a floor grid and underlying screening treated with a long-lasting residual insecticide, will, at all natural population densities, reduce to an insignificant level the number of insects that crawl through and survive. Artificially higher densities, as in rearing rooms, may require that the doorway be fitted with sliding doors in addition. These may be arranged to switch on the air current and lights as soon as they are opened.