In this study the cross‐reactivity of an antibody raised against cell surface antigens ofAlexandrium minutumHalim, a dinoflagellate isolated from the Bay of Plenty, New Zealand, during the 1993 toxic shellfish outbreaks, was tested on other strains of the same species isolated from a variety of locations, as well as a wide range of unrelated phytoplankton using an indirect immunofluorescence staining technique. The antibody showed positive reactions to all four isolates of A.minutumfrom different localities in New Zealand and one from South Australia. The antibody also showed a positive, but weaker, reaction toA. ostenfeldii(two isolates) from New Zealand but no reaction withA. lusitanicum, a species which is morphologically similar toA. minutum. No cross‐reactivity was observed with 51 species or strains from 10 different algal classes tested. The antibody bound to cells ofA. minutumpreserved with either 2.5% glutaraldehyde or 3.5% formaldehyde. Recognition of cells in stationary growth phase, including the encysted form, however, was generally weaker than for cells growing exponentially. The cell surface location of the antigen was confirmed using confocal laser scanning microscopy.