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A fish kill incident was reported at the marine fish culture areas north of Perak and south of Penang, Malaysia, on 26 May 2020. An investigation was carried out at 10 stations in Kerian, Perak and Seberang Perai Selatan, Penang. Seawater samples were collected to identify microalgae species and determination of dissolved inorganic nutrients. The physical parameters of water such as salinity, pH, temperature and dissolved oxygen were measured in situ while the dissolved inorganic nutrients were analysed spectrophotometrically. The dominant microalga was identified as Margalefidinium fulvescens (Iwataki, Kawami & Matsuoka) Gómez, Richlen & Anderson, 2017, based on the morphological and molecular characterisation of the large subunit ribosomal gene. Long rounded and ellipsoidal cells, 30-43 µm in length, appeared in chains of single, two, four or eight cells. The sulcus was slightly narrow surrounding the cell about one turn, but the cingulum was rather deep, encircling the cell approximately twice, and the chloroplasts were brownish, granular and scattered peripherally. The highest M. fulvescens cell counts were recorded at 6.22 x 105 cells L-1 and 4.61 x 105 cells L-1 in Kerian, Perak and Seberang Perai Selatan, Penang, respectively. The physical parameters of the seawater from the affected sites were within the Malaysian Marine Water Quality Standard (MMWQS) for aquaculture. However, slightly higher levels of nitrate, phosphate and ammonia were noted at several stations. Although the exact cause of the bloom was undecided, it could be due to nutrient discharge along the coasts, which also concurred with the transition phase of the northeast to the southwest monsoon. [ABSTRACT FROM AUTHOR]