AbstractCochliophilin A (5‐hydroxy‐6,7‐methylenedioxyflavone), a root releasing host‐specific plant signal triggers chemotaxis and subsequent morphological changes in pathogenic Aphanomyces cochlioides zoospores before host penetration. The present study illustrates time‐course changing patterns of cytoskeletal filamentous actin (F‐actin) organization in the zoospores of A. cochlioides during rapid morphological changes (encystment and germination) after exposure to cochliophilin A. Confocal laser scanning microscopic analysis revealed that F‐actin microfilaments remained concentrated at ventral groove and diffusely distributed in peripheral cytoplasm of the zoospore. These microfilaments dramatically rearranged and changed into granular F‐actin plaques interconnected with fine arrays during encystment. A large patch of actin arrays accumulated at one pole of the cystospores just before germination. Then the actin plaques moved to the emerging germ tube where a distinct cap of microfilaments was seen at the tip of the emerging hypha. Zoospores treated with an inhibitor of F‐actin polymerization, latrunculin B or motility halting and regeneration inducing compound nicotinamide, displayed different patterns of F‐actin in both zoospores and cystospores than those obtained by the induction of cochliophilin A. Collectively, these results indicate that the host‐specific plant signal cochliophilin A triggers a dynamic polymerization/depolymerization of F‐actin in pathogenic A. cochlioides zoospores during early events of plant‐peronosporomycete interactions. Cell Motil. Cytoskeleton 2008. © 2008 Wiley‐Liss, Inc.