A novel free-living heterotrophic stramenopile, Platysulcus tardus gen. nov., sp. nov. was isolated from sedimented detritus on a seaweed collected near the Ngeruktabel Island, Palau. P. tardus is a gliding flagellate with tubular mastigonemes on the anterior short flagellum and a wide, shallow ventral furrow. Although the flagellar apparatus of P. tardus is typical of stramenopiles, it shows novel ultrastructural combinations that are not applied to any groups of heterotrophic stramenopiles. Phylogenetic analysis using SSU rRNA genes revealed that P. tardus formed a clade with stramenopiles with high support. However, P. tardus did not form a subclade with any species or environmental sequences within the stramenopiles, and no close relative was suggested by the phylogenetic analysis. Therefore, we concluded that P. tardus should be treated as a new genus and species of stramenopiles and have proposed a new family, Platysulcidae fam. nov., for this phylogenetically distinct organism. (C) 2015 Elsevier GmbH. All rights reserved.