Cryomicrotomy and immunofluorescence microscopy employing three different categories of monoclonal antibody (MAb) that label antigens on the surface of one or both flagella ofPhytophthora dnnamomi have been used to follow the synthesis and assembly of flagellar surface components. MAb Zf 1 binds to the surface of both the anterior tinsel and posterior whiplash flagella, as well as to a nuclear component. The labeling of the flagella is punctate in nature, is brighter at the flagellar base, and does not always extend to the distal tip of the flagella. MAbs in the Zt group recognise an antigen that is located along the sides of the tinsel flagellum and may be associated with the base of the mastigonemes. Immunodot-blot analysis has shown that binding of Zt MAbs is abolished by pretreatment with either pronase or periodate oxidation indicating that the antigen is a glycoprotein. MAbs in the Zg group bind to the mastigonemes on the tinsel flagellum and to packets of mastigonemes in the cytoplasm of zoospores. Zt and Zg antigens increase in abundance during zoosporogenesis and are present throughout the life cycle of the fungus, whereas the non-nuclear localisation of the Zf antigen appears only during sporulation. Prior to association with the flagellar surface, all three components become clustered in the groove region of zoospores. They do not become associated with the flagellar surface until at least 15 min after the flagellar axoneme has formed.