Several factors suggest that this bloom originated in the open sea. One possible cause of the event could be the current climate anomaly during the last 14 months in the region, with drought conditions and strong sunshine. These factors produced a 1.5 ºC increase in the surface water temperature, in the surrounding area (15ºC). At the time of the bloom, the toxicity and the allelopathic properties were studied in the contaminated seawater under laboratory conditions. The toxicity was evaluated using a haemolytic test and the allelopathic activity was studied on two species, a diatom (Leptocyndrus minimus ) and a dinoflagellate (Alexandrium catenella ). Both of these produce red tides in southern Chile, and the Gymnodinium sp. density in the allelopathic experiment was 800 cell/ml. The haemolytic activity was very high with Gymnodinium sp. concentration of 4.000 cell/ml. A diluted sample (250 cell/ml) still produced significant activity, with 18% haemolysis. In the allelopathic experiments, the growth of the diatoms was totally inhibited in water containing Gymnodinium sp, while the growth of A. catenella was practically not affected. When this seawater was passed through Sep-Pak and Florisil cartridges, the inhibitory effect on the diatom growth was suppressed. This behavior is similar to other ichthyotoxic species of Gymnodinium present in other parts of the world. The biological properties of this species, along with the punctual climatic and hydrological conditions, facilitated the development of this bloom which had the largest geographical coverage ever recorded in Chile.