The ultrastructure of the secondary zoospores ofAphanomyces astaciandA. laeviswas compared. The general appearance of the organelles and their compartmental–ization is the same, but some subtle differences were found.A. laevishas a less distinct distribution of fuzzy vesicles around the border of the water expulsion apparatus thanA. astaci.The content of the U–body of theA. astacizoospore is paracrystalline but theA. laevisU–body has a tubular content. In the peripheral cytoplasm of both species are seen vesicles with a granular content. Flattened cisternae are found in a narrow zone just below the plasmalemma. These two structures are confined to the zoospore stage of the fungi. The lipid–microbody complex ofA. laeviscysts is not present in the zoospore. The ultrastructural organization of uniflagellate and bi–flagellate zoospores is
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