The success of Phaeocystis and its im-pacts on marine ecosystems have been linked to their capacity to form large gelatinous colonies [1]. Phaeocystis colonies produce hemolytic substances, and are responsible for clogging fishing nets, repelling fish, for oxygen deple-tion, and for negative impacts on ben-thic life. These blooms affect tourism and recreational activities due to the water discoloration and deposition of thick layers of odorous foam on beach-es [2].They also produce dimethyl sul-phide, which produces cloud-condensa-tion nuclei, increasing cloud cover and affecting regional climates [3]. During late March 2012, a bloom of Phaeocystis sp. occurred in Cayo Largo del Sur, a small cay in Los Canarreos archipelago, on the south-western Cu-ban platform (Fig. 1). This site is highly hydrodynamic coast. A red-brown wa-ter discoloration persisted for 15 days. The bloom occurred within 200 m of the shore and extended along 22 km of coast. As well as discoloration, abun-dant foam and mucilage were observed (Figs. 2-5). Seawater was sampled for phyto-plankton and physico-chemical analy-ses. The ratios Dissolved Inorganic Nitrogen/Dissolved Inorganic Phospho-rus and Total Nitrogen/Total Phospho-rus were calculated. The algal samples were studied using a light microscope Laborlux, Leica-Leitz with phase con-trast equipped with Motic digital cam-era. We observed large colonies formed by cells embedded in a firm gelatinous matrix. Cells are slightly kidney-shaped