A study was made of the vertical variability of an assemblage of diatom and thecate dinoflagellate species in the Gulf of Arauco ( Chile), from 17th to 27th january, 1985. Sampling was carried out approximately every 12 hours at a fixed station. Samples were obtained at 8 depth levels for the qualitative and quantitative analysis of diatoms (in terms of filaments for colonial species) and thecate dinoflagellates. Salinity and nutrients (nitrate and phosphate) were measured from water samples, and water temperatures were recorded. Results showed a stratification in the distribution of these variables in the water column. During the study period the Gulf of Arauco phytoplankton was dominated by diatoms, among which the most important due to their recurrence frequency, persistence, and quantitative contribution were: L. danicus, N. seriata, C. closterium, D. pumila, E. cornuta, S. costatum, Rh. stolterfothii, Rh. delicatula and Rh. alata. A similar taxonomic composition was observed between lm and maximum chlorophyll depth, and a high phytoplankton abundance was observed in the superficial layer (1 to 20 m depth) of the water column. The persistence through time together with the fact that 95% of total filament number belong to the 9 dominant species suggests that these few species used the nutrient supply to the euphotic zone efficiently.