The seasonal succession of marine phytoplankton at the Port Hacking 100 metre coastal station was studied using living algae. The spring (September) bloom began with small chain-forming diatoms, mainly Asterionella, Thalassiosira, Skeletonema, Nitzschia and Chaetoceros species. These were succeeded by larger centric diatoms, the most abundant of which were species of Ditylum, Leptocylindrus, Eucampia, Rhizosolenia and Melosira, together with the pennate genus Thalassiothrix. Coccolithophorids, microflagellates (probably mostly chrysomonads) and dinoflagellates showed a first minor pulse in September, and became established in October and November. During late summer and early autumn, when diatom numbers were very low, the phytoplankton consisted mainly of dinoflagellates, coccolithophorids and microflagellates. A pulse of cryptomonads and silicoflagellates occurred in March and April respectively. A second diatom flowering took place in the autumn (May), dominated by species of Leptocylindrus, Thalassiothrix, Rhizosolenia etc., which were different from those which initiated the spring (September) bloom. Clear succession patterns were evident, in principle very similar to successions observed in temperate coastal waters in other parts of the world. These patterns may correspond, not to gross mineral enrichments, but to specific vitamin interrelationships among the algae.