The rotifer Asplanchna sieboldi performs several well-defined behavioural responses to external stimuli. Contact of male and female occasionally elicits a mating response in the male. The mating response is composed of a series of distinct behavioural elements. After the initial contact, the male stretches out its body wall outgrowths, arcs into a characteristic mating posture and glides along the surface of the female until the penis becomes attached to the female’s body wall, through which the spermatozoa are injected. Under experimental conditions, the mating response could be evoked by stimulation with agar spheres that were impregnated with extracts prepared from females, while agar spheres prepared from buffer only did not cause any mating response. If the active substance was added directly to the suspending medium in the observation chamber, mating could be triggered just by mechanical stimulation. It is concluded that the mating response is triggered by a combination of chemical and mechanical stimuli. During the time course of the experiment there was no adaptation to the chemical stimulus. The receptor cells involved in the perception of the two stimuli remain to be identified. The mating response suppressed other behavioural responses that are usually triggered by tactile stimulation of the apical mechanosensors, suggesting that signal processing by the nervous system occurs.