Ultrastructural observations on the vegetative filaments, motile cells and processes leading to zoospore differentiation in Chrysonephos lewisii are presented. Vegetative filaments are embedded in a mucilaginous envelope, preserved by Alcian blue fixation, which favours their aggregation and also the attachment of bacteria, diatoms and various types of debris. The filaments are provided with an external wall consisting of microfibrils positive to the PATAg test and ConA-colloidal gold labelling. Inside the filaments the vegetative cells differentiate into zoospores. This accounts for the ultrastructural similarity between vegetative cells and zoospores, although zoospores are typically provided with a flagellar apparatus. They share an external cell covering or theca, vacuolar or cytoplasmic ‘scale-like structures’, protruding stalked pyrenoids, scattered nucleoids of plastid DNA and absence of a photoreceptor-eyespot complex. The flagellar apparatus of zoospores is characterized by two distinct basal plates, the central pair of flagellar microtubules originating some distance above the transitional plate and basal body angle less than 90°. These results provide strong taxonomic evidence that this genus should be placed in the Sarcinochrysidales sensu stricto, rather than remaining in Chrysomeridales. The key role of the Golgi apparatus in the synthesis of different structural elements – depending upon the functional stages of the cells – and in the processes of exocytosis during zoosporogenesis is also discussed.