Abstract
On the south coast of Buenos Aires Province, Argentina, extensive blooms of diatoms were observed in the surf zone of a sandy beach. Gonioceros armat us (West) Peragallo & Peragallo was the most important species of the bloom (98% of the population). The distribution and abundance of the species together with the physical characteristics of the area related to the bloom have been studied. This is the first record of the occurrence of G. armatus in South America.