Mass mortalities of intertidal purple sea urchins, <strong></strong>Strongylocentrotus purpuratus<strong></strong> occurred at Malibu Lagoon State Beach, California, in 2010 and 2011. Both events followed the first heavy rain of the season, and coincided with an illegal breaching of the lagoon. Osmotic shock from low-salinity lagoon water, the likely cause of death, may have acted jointly with stress from exposure during especially low tides. Comparable die-offs of purple sea urchins have occurred at other localities after exposure to brackish water or thermal shock. Annually recurring lagoon ruptures at Malibu, combined with predation by western gulls, can have a profound impact on the local population of <strong></strong>S. purpuratus<strong></strong> and on intertidal ecology.