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The toxic marine dinoflagellate Alexandrium minutum that associated with paralytic shellfish poisoning is widely distributed in the coastal waters of Asia Pacific region. In early September 2015, a remarkably high density of A. minutum occurred in Sungai Geting, a semi-enclosed lagoon situated at the north-eastern Peninsular Malaysia, causing severe discoloration and contaminated the benthic clam, Polymesoda similis. Plankton and water samples were collected to investigate the mechanisms of blooms initiation, development and termination of this toxic species. Flow cytometric analysis of the plankton samples revealed that the bloom was likely initiated by cyst excystment, as planomycetes (4C) were observed (88%) in the early development of bloom. Increase in planozygotes (2C) was signified at the middle of the bloom, coincided with an abrupt decrease in salinity and elevated N:P ratio, that might induce gamate expression. Flow cytometric results confirmed that the bloom was initiated at the inner part of the lagoon and slowly dispersed to the river mouth as 4C cells were found dispersed from the inner part to the outer part of the lagoon. The bloom was sustained by the vegetative cells (72  92%) for almost two months. Increase of precipitation and freshwater intrusion from the Golok River plumes into the lagoon might cause the bloom to terminate. This event provides a fundamental understanding of the life cycle and the bloom dynamics of this toxic dinoflagellate. Keywords: Paralytic shellfish Poisoning (PSP), flow cytometry, cyst, planomycetes, planozygotes