In summer 2007, a dinoflagellate preliminarily identified as Protoceratium reticulatum bloomed in Bahia Mejillones, northern Chile. Phytoplankton samples were analyzed in detail by light and scanning electron microscopy revealing the presence of resting cyst and motile cells of P. reticulatum. Oceanographic and phytoplankton data suggest that the bloom was initiated offshore by motile cells and germinated cysts during an upwelling pulse. These cells were advected into the bay when upwelling relaxed and grew without any relevant competitor. Phytoplankton net samples were found to contain yessotoxin as the only toxin in an estimated proportion of 0.2 and 0.4 pg cell(-1), thus confirming that P. reticulatum is a source of yessotoxin in northern Chilean waters and consequently that it poses a risk for human health and mollusk exploitation in the area. (C) 2011 Elsevier B.V. All rights reserved.This research was funded by the projects DOI 1056 (FONDEF-CHILE) and FIP 2007-20 developed within the framework of a cooperation agreement between the Conselleria do Mar, Xunta de Galicia, Spain, and the Universidad Catolica del Norte, Chile. We would like to thank to Dr. Santiago Fraga by his valuable comments. We would also like to thank Ines Pasten and Paulo Avalos, for their technical assistance. We also acknowledge the Asociacion de Productores de Ostion y Ostras de Chile (APOOCH) for their support. We are grateful to Jesus Mendez and Ines Pazos by its technical assistance in SEM. Gonzalo Alvarez was funded by a MAEC-AECI grant.5FONDEFgalvarez@cimacoron.orgFONDEF-CHILE [DOI 1056]; Conselleria do Mar, Xunta de Galicia, Spain [FIP 2007-20]; Universidad Catolica del Norte, Chile [FIP 2007-20]; Asociacion de Productores de Ostion y Ostras de Chile (APOOCH); MAEC-AECI4FONDEF65