Lanthanum was used as an electron-dense marker of apoplastic transport in the brown algaCystoseira nodicaulis. A lanthanum salt, La(NO3)3, was given in seawater at the base of excised branches for 2–8 days. Lanthanum transport into two regions distant from the point of application, the base and apex of the branches, was followed over time by electron microscopy. Restricted localisation of the deposits confirmed that an apoplastic pathway exists in this alga. The kinetics of transport are slow. Dense deposits were located exclusively in the cell walls of meristoderm and cortex cells. The significance of apoplastic transport in algae is discussed in the context of long-distance transport. The existence of an apoplastic route does not exclude the occurrence of symplasmic transport, which is suggested by the numerous plasmodesmata present in the medulla.Key words: algae, apoplast,Cystoseira, lanthanum, long-distance transport.