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Cyprid larvae of Balanus cariosus settle preferentially on slate plates with a biota characteristic of the lower intertidal shore, and the cyprids also prefer plates with more algae. Cyprid larvae of Balanus glandula had the same preferences in two out of three experiments. We conclude that some component of the flora guides both species during settling and metamorphosis. Data on vertical distribution and fecundity of B. glandula show that the preference for the lower shore decreases fitness of B. glandula at the site of the settling experiments and at most other sites sampled in or near the San Juan Islands, though in some restricted habitats in the San Juans and extensive areas in the adjacent regions of Puget Sound a preference for the lower shore is appropriate. Extensive dispersal among sites is possible in the planktonic period of 2 to 4 weeks. This example supports the hypothesis that a cost to large scale dispersal is lower fitness at many sites within a species' range. In this case the cost is through poorer correlation between stimuli guiding choice of habitat and favorability of habitat.

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