||LIGHT TRAPS of various forms have been used to collect and study moths for well over 100 yr, but surprisingly little is known about how they attract moths. There has been some evaluation of the factors influencing the size of light trap catches1â5 and of the mechanics of the terminal phase of the moth's approach to a light6, but virtually nothing is understood about the light-trap response itself. Such an understanding is perhaps unnecessary when light traps are used solely to collect specimens, but becomes crucial as soon as they are used for quantitative sampling or survey work7. Of particular importance to the interpretation of such work is a knowledge of the distance from which moths orientate with respect to a light source; it seems intuitively that this distance should be fairly large. We present here the results of three experiments designed to determine the distance of response of free-flying moths to an artificial light source. Our results support Sotthibandhu's claim4 that the effective range of a 125 W mercury vapour (MV) lamp is about 3 m. They also lead to speculation concerning the behavioural meaning of the light trap response in moths.