The symbiotic scale worm polychaete Arctonoe vittata mainly exploits the limpet Niveotectura pallida and the sea star Asterias amurensis as hosts in southern Hokkaido, Japan. Its size distribution was different between the two host species, in which smaller individuals were often observed on the sea star, while larger ones were in the mantle cavity of the limpet. The host exploitation pattern of the scale worm was examined by several experiments. Scale worms infesting the sea star and the limpet showed significantly higher survival rates than those detached from the hosts under a risk of crab predation. Larger scale worms showed significantly higher survival rates in the limpet than in the sea star. In competition trials for the limpet host, relatively larger scale worms won and smaller ones sometimes died probably due to severe struggle. These results suggest that the scale worm switches its host species from the sea star to the limpet as it increases in body size, because smaller individuals improve their survival rate in the sea star, while larger ones are able to guard their host limpet from conspecifics and improve their chances of survival. The achievement of optimal host exploitation in Arctonoe vittata therefore seems to be closely correlated to ontogenic growth.