AbstractSporangia and zoospores of the oomycetePhytophthora infestansare characterized by a number of tubular cytoskeletal elements: Single and bundled cytoplasmic microtubules, arising from electron dense material at the centrioles, form a network inside the zoospores of the fungus. Nucleus‐associated microtubules surround the nucleus in a basket‐like manner. Spindle microtubules occurring during mitosis in zoosporangia are partially maintained from the late telophase till the zoospore stage. The pattern of flagellar microtubules is identical to that of the typical eukaryotic flagella. The bundles of 15 nm‐tubules observed in ER‐cisternae of zoosporangia and zoospores are similar to preformed mastigonemes of tinsel flagella because of their dimensions. Microtubules, flagellar fragments, and mastigonemes, similar in substructure to those of other lower and higher eukaryotes, have been isolated from zoospores ofPhytophthora infestansby lysis in Triton X‐100 on microtubule‐stabilizing