The relationship between known densities of house flies,Musca domesticaL., and fly-abundance indices obtained with baited jug-traps and white spot cards in a caged-layer poultry house was determined. Baited jug-traps hung from the ceiling and spot cards fastened to feed troughs gave fly-abundance indices most sensitive to changes in house fly density; spot cards fastened flush against the rafters were slightly less sensitive, and spot cards hung vertically from the rafters were least sensitive. With increasing mean daily temperature (range, 22 to 34°C), estimates of fly-abundance density obtained with baited jug-traps, spot cards fastened to the feed trough, and spot cards fastened hanging from the rafters increased, while the estimates obtained with spot cards fastened flush against the rafters decreased. The relationship between fly-abundance indices obtained with baited jug-traps and spot cards (in different locations) was determined by linear regression. Equations are given for calculating the numbers of sampling devices required at different fly-abundance thresholds for obtaining indices with a desired coefficient of variability.