The association of bacteria with dinoflagellates has been a neglected field of study, which has gained prominence in recent years because of the possible role of bacteria in toxin synthesis. A number of dinoflagellates undergo sexual reproduction, passing through various life-cycle stages in addition to the vegetative form. The presence of bacteria within dinoflagellates has been well established, but their presence throughout the dinoflagellate life-cycle has not been investigated. Using cultures of Alexandrium (A. tamarense, A. fundyense), we investigated the association of bacteria with various vegetative growth phases (lag, log, stationary) and sexual life-cycle stages (planozygote, planomeiocyte, hypnozygote), using scanning electron microscopy, transmission electron microscopy (TEM) and epifluorescence microscopy. Bacteria were found to be associated with the surfaces of vegetative cells, planozygotes, hypnozygotes and planomeiocytes. TEM showed bacteria to be present within all vegetative growth phases, as well as in the sexual planozygote, cyst and planomeiocyte. The presence of intra-cellular bacteria in vegetative cells was also confirmed using DAPI staining combined with epifluorescence microscopy, and lipopolysaccharide staining combined with TEM.