The dinoflagellate Haidadinium icthyophilum gen. nov. et sp. nov. is associated with the threespine stickleback, Gasterosteus aculeatus L., in freshwater. This new genus differs from all previously described dinoflagellates in the morphology and ultrastructure of its complex life history stages and in the ecology of its interaction with G. aculeatus. Distinguishing characters included (i) the absence of thecal plates and the occurrence of chloroplasts in the short lived swarmer (=dinospore) stage; (ii) the development of four distinct amoeboid stages, including a spheroidal, rolling amoeba unknown in any other species; and (iii) the fact that this dinoflagellate causes epithelial hyperplasia in the stickleback and does not result in massive fish kills. Haidadinium icthyophilum is known only from two acidic lakes in the Queen Charlotte Islands, British Columbia, Canada. Haidadinium is tentatively assigned to the family Phytodiniaceae of the order Phytodiniales.  

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