The marine brown alga Myriotrichia clavaeformis (Dictyosiphonales) is the host of a large icosahedral virus with a double-stranded DNA genome, MclaV-1. This pathogen replicates exclusively in prospective reproductive organs of the alga. We studied the assembly of virus particles by transmission electron microscopy. Replication of viral DNA starts in the nucleus, which hypertrophies and later disintegrates. Capsid assembly begins in the cytoplasm by budding from virus-detaching bodies. After nuclear breakdown, assembly continues in a mixed cyto-/nucleoplasm on membrane cisternae, which probably originate from the endoplasmic reticulum. Virus particles thereby acquire as an integral capsid component a membrane to which proteins are apposed. Material inside the capsids partly condenses to form an additional layer in the core shell. DNA is packaged after capsid formation, giving rise to an electron-opaque nucleoprotein core. M. clavaeformis infected by MclaV-1 is the second brown algal host-virus system in which virus assembly has been studied in detail. Together with previous observations, our results allow conclusions on general mechanisms of virus assembly in brown algae. Some features of virus formation in brown algae show similarities with large icosahedral DNA viruses infecting animal cells.