In laboratory experiments, the swimming behaviour of the ectoparasite Argulus foliaceus and its infection rates on juvenile perch (Perca fluviatilis) and roach (Rutilus rutilus) were examined. The highest infection rate and a preference for perch juveniles were obtained in darkness, the lowest infection rate and a lack of preference in the light, when aquaria with glass walls (high reflectivity) were used. In the light, when aquaria were lined with black plastic (low reflectivity) an intermediate level of infection for perch and the highest for roach was recorded. Under such conditions roach were significantly more heavily infected than perch; an attack rate 4 times greater was recorded for brighter (more reflective) roach juveniles than for perch. Within the aquaria with a low reflective interior parasites swam 4·4 times slower and were observed predominantly in the central area, while in the highly reflective aquaria fast swimming A. foliaceus were recorded mainly near the walls. The primary role of visual stimuli for the host search behaviour of A. foliaceus in the light is suggested. Parasites can effectively use such stimuli only in the low reflective surroundings. Highly reflective glass aquarium walls produce numerous secondary local light sources, which cause fast, erratic parasite movements and prevent the efficient location of potential hosts.