The phytoplankton population dynamics of 3 selected localities of the north-west section of the Arab Gulf was studied in Oct. 1975. Station I was the richest of all, followed by station II; station III was the poorest. The mean number of phytoplankton in station I was 4.42 .times. 106/m3 and the estimated wet weight, dry weight and productivity were 3.7588 g/m3, 0.4898 g/m3 and 5.9 mg C/h/m3, respectively. The phytoplankton species composition was dominated by diatoms as opposed to dinoflagellate dominance in the Arabian Sea. At the time of sampling there was a bloom of Rhizosolenia and Lauderia whereas station III was at the extreme border. During the night of the sampling the whole water of station III was replaced by a new mass of water coming possibly from the western basin. The species composition of the new water mass was dominated by Navicula. The biomass and the productivity of the watermass of station III remained unchanged. A Navicula species was observed as an epiphyte on Lauderia in the shallow waters of station I and station II.