The impact of interspecific behavioral differences on the relative susceptibility of third instar Aedes aegypti and Anopheles quadrimaculatus larvae to infection with cercariae of Plagiorchis noblei was determined. When permitted to move freely in a column of water, larvae of Ae. aegypti were significantly more susceptible to infection with the parasite than were An. quadrimaculatus larvae. This difference is ascribed to the significantly greater activity of Ae. aegypti larvae in the water column. Since cercariae are suspended in the column, particularly near the bottom, contact with larvae of Ae. aegypti may be enhanced, whereas contact with An. quadrimaculatus larvae, which tend to remain near the surface, may be reduced. Interspecific differences other than behavior are not thought to play a major role, since immobilized larvae of the two species did not differ significantly in their susceptibility to this parasite.