Harmful Algal Blooms (HABs) can have a significant impact on the distribution and survival of coastal fishes and invertebrates, and consequently they can affect local economies where fishing is an important activity. In October 2005, extensive algal blooms with brownish/orange discoloured water and fish mortalities were observed east of Masirah Island. Satellite images revealed cooler upwelled surface water along a broad front just prior to the event, followed by a gradual warming period coinciding with the mortalities. Depth profiles of dissolved oxygen (DO), temperature, salinity and pH taken on 19th October (during the fish mortality event) showed a pronounced thermocline at ~15 m depth and minimum DO of 0.82 ml/L at 25 m depth, and a slight improvement in DO to 1.3 ml/L was measured on 22nd October (after the event). Demersal fishes of several families were prominent among mortalities. No bacterial infestation was found in fish samples and no human poisoning was reported. Planktological data showed that marine dinoflagellates Noctiluca scintillans and Prorocentrum micans and toxic blooms of cyanobacterium Trichodesmium erythraeum were present.