Traditionally in Australia, regular applications of insecticide to the floors and lower walls of broiler houses after cleanout periods have been used in an attempt to control lesser mealworm, Alphitobius diaperinus (Panzer). The Australian chicken meat industry has been concerned in recent years with the failure to control A. diaperinus in its broiler houses by using this method and with large beetle populations breaching farm biosecurity. Resistance to fenitrothion was suspected to be responsible for these recent control failures. In response, beetles from 13 poultry facilities were compared with an insecticide-susceptible reference population by using a topical application method. Generally, strong resistance to fenitrothion (up to 79 times that of the susceptible at the LC₅₀) occurred in populations of A. diaperinus in long-established broiler growing areas of southeastern Queensland, where fenitrothion had been used continuously for up to 20 yr. In newly established broiler growing areas, where considerably less fenitrothion had been used (i.e., ≈5 yr), much weaker or no resistance occurred. In addition, dose–mortality data generated for the susceptible reference beetle population over a range of fenitrothion concentrations showed that 0.15% fenitrothion at a LC_99.9 level could be used as a convenient topical dose concentration to discriminate between susceptible and resistant individuals. Using this method, the susceptibility of 27 field populations of A. diaperinus was determined. Of this total, 23 populations did not exhibit complete mortality against the discriminating concentration (mortality range 0–98.7%). Application of fenitrothion in Australian broiler houses for control of A. diaperinus has now ceased.