Determinations on the penetration of ultra-low volume applied insecticide into dwellings was accomplished with a model house in Frederick, MD, USA, and native houses in Santo Domingo, Dominican Republic. Results from the model house tests show that aerosol droplets, with a volume median diameter of 4 microns, penetrated and remained suspended in low recesses of the building during the first 2 min after spraying. Similar results were found with tests in Santo Domingo, where man-made or natural obstructions were determined to be critical factors in aerosol penetration.