Abstract Stainless steel panels (15.2 cm2) were contaminated with cockroach frass (i.e., cockraoch feces, secretions) by exposing the panels to adult German cockroaches. Five stainless steel panels, separated by wooden spacers (0.48 cm in diameter) and held together by rubber binders, were placed vertically in plastic tubs containing approximately 1000 cockroaches of mixed sex. Cockroaches and panels were housed together at 28 ° 1°C in a 12:12 (L:D) photocycle, were provided Wayne Rodent Blox* and water, and after 25 d the panels were removed. The quantity of frass on each surface was not determined. Dilute aqueous sprays of 0.05% cyfluthrin (Tempo 20 WP and Tempo 2 E) and 0.5% chlorpyrifos (Dursban 2 E and Empire 20) were applied to clean and frass contaminated panels by use of a spray tower with a mechanized nozzle assembly (Spraying Systems Tee-Jet 8001 E). The spray tower was calibrated to deliver 0.5 ml/232 cm2 of cyfluthrin and 0.95 ml/232 cm2 of chlorpyrifos. The panels were aged in the laboratory (25°C and 50% RH) prior to bioassays 1 and 28 DAT. Ten adult cockroaches (5 male, 5 female) from a non-resistant laboratory colony were used for bioassay. A 12.7 cm diameter Plexiglass ring, lightly greased with petrolatum/mineral oil mixture, confined the insects to the panels during a continuous exposure. Knockdown was recorded at 4, 5, 6, 7, 8, 9, 10, 11, 12, 14, 16, 18, 20, 22, and 24 min for cyfluthrin treatments; 15, 30, 45, 60, 75, 90, 105, 120, 135, 150, 165, 180, 195, and 210 min for chlorpyrifos treatments. Probit analysis determined KT50 and KT95 values (based on the response of 3 replications, 30 insects) for each insecticide and surface combination.