Results are presented on the phytoplankton species composition and abundance from bottle samples collected in September 1989 near the confluence of the Brazil and Malvinas currents off Argentina. The phytoplankton assemblages were dominated by diatoms and dinoflagellates. A surface diatom bloom was found along the west side of the Brazil Current, and was dominated by Thalassiosira delicatula Ostenfeld emend. Hasle (cell numbers up to 5.5 × 105 cells 1−1) The bloom was associated with strong temperature gradients separating Brazil and Malvinas waters, and with the presence of a cyclonic eddy near the confluence of the currents. These features were detected in satellite imagery coincident with the in situ sampling dates.