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Epiphytic diatoms associated with three species of seaweeds, the phaeophyte Bachelotia antillarum, the ulvophyte Caulerpa verticillata, and the rhodophyte Haloplegma duperreyi, were investigated in the subtidal zone of Northeastern Brazil. A total of 54 diatom species were recorded, distributed among motile (24 species), erect (20 species), and adnate growth forms (five species), in addition to accidental occurrences (five species). Although motile diatoms showed the highest number of species, erect growth forms were the most abundant, accounting for 69% of the entire epiphytic diatom cell abundance. The total abundance ranged from 29,783 to 45,842 cells g-1 wet weight (ww), with Melosira moniliformis (erect), Rhabdonema adriaticum (erect), Cocconeis scutellum (adnate), and Surirella fastuosa (motile) as the most abundant species. The results confirmed qualitative and quantitative differences between the hosts with a greater shape diversity in B. antillarum whose thalli (filamentous-like tufts) offered an increased surface area for colonization mainly by epiphytic erect growth forms. [ABSTRACT FROM AUTHOR]