According to monitoring studies carried out by the National Fisheries Institute in cooperation with the Air-Naval Military Base Capitán C. Curbelo in Uruguay several algal blooms were detected, causing changes in the water color. In the present work we describe these phytoplankton species which produce discoloration in Uruguayan coastal waters and their temporal and spatial distribution. Water discoloration events are mainly caused by dinoflagellates and diatoms, as well as by cyanophytes and ciliates. Most of the species found in this study were dinoflagellates that appear in spring-summer (November-March), during calm sea and soft wind conditions. In February 1994 a dinoflagellate bloom of Gyrodinium sp. and Gymnodinium catenatum Graham caused toxicity and water discoloration due to their high abundance (4 × 107 cells.l-1). Most discoloration events were caused by non-toxic species and harmless, although they may cause public alarm because of their possible association with mollusc toxicity.