An Insect Activity Monitor was created to measure the behavioural responses of fleas (Siphonaptera). The apparatus allows for a range of visual, chemo- and mechanoreceptor cues to be presented. The jumping response is detected by counting amplified pulses produced as the fleas land on a stretched membrane held over a microphone. Horizontal movements are detected using a system of infra-red beams and phototransistors which, when broken, are counted as a measure of activity. The apparatus was tested using Ceratophyllus hirundinis (Curtis), Ceratophyllus farreni (Rothschild) and Ceratophyllus rusticus (Wagner), co-inhabiting species from the nests of the house martin (Delichon u.urbica L.). No unaided emigration or immigration has been demonstrated in these species (Clark, 1988) and much of their time is spent confined to the darkness of the nest. In addition to species contrasts, females were more active than males and fleas were more active at 25 degrees C than at 18 degrees C. The more active bird flea Ceratophyllus garei (Rothschild) from the nest of pheasant (Phasianus colchicus L), was tested for responses to light of varying intensity. Activity was stimulated by white light, but not proportional to light intensity.