The choice between two alternative hosts, brown trout (Salmo trutta) and roach (Rutilus rutilus), and the response to visual and olfactory cues were studied in the ontogeny of Argulus coregoni. The initial preference of the smallest parasites for brighter roach changed at the age of 2 weeks, at the size of about 2 mm, for trout, a typical salmonid host. Younger argulids were attracted by a non-specific visual stimulus (white disc over dark background), and they did not respond to olfactory stimulation (fish-conditioned water). Later, the response to visual stimuli was modulated by trout-conditioned water, but not by that conditioned by roach. The primary role of vision, particularly in early ontogeny, is emphasized as an adaptation of A. coregoni to habitats in boreal latitudes, clear and running water with a sparse fish population. In sub-adult and adult parasites, vision is complimented by olfaction that enables them to choose hosts more precisely. The nature and adaptive significance of the ontogenetic shift in host choice by A. coregoni is discussed.