Investigations on phytoplanktonic communities in relation to the thermal vertical structure of the water masses, in the Cariaco Trench during 1982, revealed the existence of significant differences throughout the year. Two seasons, dry and rainy, and a transition period between them were distinguished. During the dry season, relatively low temperatures (mean = 22.3°C) were observed between surface and 100 m depth, due to upweling. As a result, high biomass (0.65 mg chlorophyll a/m3), developed. Diatoms were the dominant organisms during this period. Thermal stability increased during the transition period, while biomass decreased (0.35 mg chlorophyll a/m3). Diatoms and dinoflagellates became codominant organisms. The highest mean temperature (25.0°C), in the area occurred during the rainy season; strong thermal stability and low biomass (0.26 mg chlorophyll a/m3), also occurred. Flagellates and blue-green algas were present in the surface water during this period. At some localities, the highest density of organisms and maximum biomass occurred below 30 m depth, associated with the thermocline or below it.