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During the late austral summer and early autumn 1995 (March-April), an oceanographic cruise was conducted along the Strait of Magellan in order to study the photosynthetic performance of phytoplankton assemblages. The high correlations between the pico-nano fractions and both the total biomass concentrations (Chl a) and primary production rates emphasized the role of these fractions in driving the primary production processes. Repeated P versus E experiments were conducted in the most productive area of the Strait, Paso Ancho, in order to assess the influence of the tidal currents on phytoplankton photosynthetic performance. These data were compared to those available from a previous cruise (February-March, 1991) carried out along the Strait. In the Pacific-Andean sector, the primary production processes were highly controlled by wind, land forcing, and irradiance availability. In the Paso Ancho, the observed highest photosynthetic capacity $$ P_{\text{m}}^{\text{B}} $$ (up to 6.5 mgC mgChl a h) and the high primary production rates may due to the continuous mixing of the water column forced by the strong tidal currents within the photic layer. The non-limiting, macro-nutrient concentrations in the Strait indicate that the available irradiance and the depth of mixed layer are the main driving factors of the primary production processes. The photosynthetic performance of the phytoplankton assemblages renders the Strait a unique ecosystem, which is more similar to those of the mid-latitudes than to those of the periantarctic areas. [ABSTRACT FROM AUTHOR]