Medusa blooms are a scarcely known phenomenon. For temperate, southwestern Atlantic waters, reports of such events are almost nonexistent. This research reports the analysis of a total of 1043 plankton samples taken from 1993 to 2006 and benthic samples collected by seasonal SCUBA diving since 1997 along the Buenos Aires coast. The medusa Obelia longissima was observed in very low densities and frequencies, except in October 2003, when a bloom of this species was detected. Soon afterward, two consecutive massive accumulations of O. longissima and Amphisbetia operculata colonies were detected on the coast by the inhabitants of Mar del Plata (38 degrees 00'S-57 degrees 32'W) and the coastal Mar Chiquita Lagoon (37 degrees 32'S-57 degrees 19'W) between March and April 2004. The colonies of both species were collected from the shoreline at low tide at both sites. The environmental factors of that period seemed to be particularly appropriate for promoting optimum growth conditions for these hydroid species. This paper reports and describes the mentioned abnormal phenomena, providing baseline information on these little-understood events.