The dynamics and biomass of the Chl a and net phytoplankton (diatoms and dinoflagellates) community and their relationship to physico-chemical parameters in Bandar Khayran Bay were investigated from monthly samplings between January 22, 2001 to January 22, 2002, at 4 stations. The parameters investigated were: temperature, salinity, turbidity, dissolved oxygen, nitrite, nitrate, phosphate, and silicate. The annual cycle of surface temperature at Bandar Khayran followed a clear cycle with maximum values during summer (34°C) and minimum values during winter (22.5°C). Even in summer, fluctuation of up to 8 °C was recorded over short periods of time. The water column was mostly homogenously distributed from January to May when stratification occurred with the development of a thermocline ranging from 5 m to 15 m from May to October, respectively. In the winter, in a more homogenous water column, dissolved oxygen was distributed about evenly in the water column and ranged from 4.5 to 6.5 mg/L. With increased stratification occurring during summer, summer months showed large variations ranging from 6.88-2 mg/L but these variations decreased during July. Nitrite concentrations at all stations ranged between 0.1 to 0.4 uM throughout the sampling periods. Nitrate concentrations ranged from 0.1 to 0.5 MM during most the study period except on February 7 when maximum concentrations ranged from 2.0 to 2.7 uM. Phosphate concentrations were at a more constant rate at 1.0 um except for station "Out", which showed an average concentration of 3.0 um during the months of April and May. The level of silicate at the Bay was almost steady with concentrations of about 2.0 um, except on August 20 and September 9 when station "East" showed an increase in concentration to 2.95 and 4.5 uM, respectively. Within the Bay station "Man" behaved differently, exhibiting higher silicate concentrations at all time reaching maximum concentrations of 7.9 on July 16. Surface Chl a remained below 1.0 ug/L throughout the sampling periods except during the period from May to September when a sharp increase in concentrations occurred. The highest concentration (3.7 ug/L) was recorded on July 16, whereas the lowest concentration at all stations (0.1 ug/L) were recorded on February 7. The profile of Chi a with depth showed that the concentrations remained below 1.0 ug/L at all depths during most of the sampling periods except from May 28 to July 16 when concentrations ranged from 3.5 to 5.0 ug/L between 5-15 m depth. Size class fractionations revealed that picoplankton (<2um) dominated the entire study area throughout the year making up on average 70% of the total biomass. The peak of picoplankton biomass at all stațions was recorded during July 16 to October 14. Net phytoplankton (>20um) contributed more than 40% to the total biomass only on Jan 22, March 12 and December 26. A total of 269 net phytoplankton taxa were identified during this study. The Dinophyta (dinoflagellates) contributed with the highest number of species (141) followed by Bacillariophyceae (diatoms) (125), Cyanophyta (cyano-bacteria) (2) and Dictyochopyceae (1). Twenty six species (4 diatoms, 20 dinoflagellates, 1 cyanobacterium and 1 dictyocha) can be considered toxic or harmful in various concentrations. Alexandrium spp, Dinophysis acumintata, Dinophysis acuta, Scripsiella trochiodea, Peridinum quinquecorne and Gymnodinium centatum are considered toxic dinoflagellat species. Tremendous fluctuations were found in phytoplankton population densities, with a minimum density of 52 cells L-' observed on May 8 and a maximum of 33,230 cells L' recorded on October 15. The diatoms flora was comprised of 65 species from the order Pennales and (53) from the order Centrales. Five genera were found to be more dominant than the others, Pleurosigma, Nitzschia, Chaetoceros, Coscinodiscus and Thalassiosira. Diatoms were minor constituents of phytoplankton populations at both "In" and "out stations as compared to dinoflagellates, which made up between 60 -95% of the total phytoplankton. The dominant species of dinoflagellates were Prorocentrum micans, Scripsiella sp., Protoperidinium sp. and Ceratium sp. These species were presented in countable amounts throughout the sampling period.