A series of seasonal, monthly and short-term samplings was conducted in open and confined waters of the Algiers area in 2001-2002 and summer 2003 in order to detect potentially harmful microalgae species (HAB). A total of 14 potentially toxic and 18 bloom-forming species were recorded, of which some were found for the first time in Algerian coastal waters: the potential toxin producers Alexandrium minutum, Dinophysis sacculus (dinoflagellates) and Pseudo-nitzschia calliantha (diatom), as well as the bloom-forming species Lepidodinium chlorophorum (dinoflagellate), Cyclotella meneghiniana (diatom) and Holococcolithophora sphaeroidea (holococcolithophorid). The latter produced a very important bloom with considerable water discoulouration. The occurrence and duration of this event in the Algiers Harbour underline the importance of this enclosed structure as a growth area for HAB species. The transport of a H. sphaeroidea bloom toward off-shore waters by an anticyclonic eddy demonstrates the effect of Algerian current mesoscale instabilities on the spatial distribution of phytoplankton communities.