Abstract Adult hide beetles, Dermestes maculatus, are strongly photonegative and tend to aggregate in dark places. Another factor which can cause their aggregation is an aggregation pheromone from their faeces. A two-choice test was used to investigate the insects' reaction to pheromone in various illumination conditions. Beetles were placed inside Petri dishes and given a choice between the dark part of the dish, containing control filter-paper strip, and the illuminated part of the dish, containing a pheromone-treated filter-paper strip. A series of choice tests was performed at each of six levels of illumination: darkness, 1, 10, 100, 1000 and 10,000 lux. The behaviour of insects tested in such circumstances proved to depend on the intensity of illumination. In darkness and in dim light most of the insects were aggregated around the pheromone-treated strip whereas the highest illumination intensity used in tests caused the aggregation of most insects in the dark part of the dish, around the control strip.