The ctenophore Mnemiopsis leidyi, a species native to estuaries and coastal regions of the western Atlantic Ocean, was first introduced into the Black Sea in the early 1980s, where it negatively affected zooplankton biodiversity and biomass, and commercial fisheries. This invasive ctenophore was first reported along the Spanish Mediterranean coast in 2009. In 2012, new blooms of this species were reported in the hypersaline and largest coastal lagoon of the Western Mediterranean basin, the Mar Menor lagoon, Spain. Sampling in the lagoon during summer 2012 showed an average abundance of 23.4 ctenophores 100 m(-3), in early August, declining to 8.2 ctenophores 100 m(-3) by early September. The population contained only adults (total length 19 to 79 mm), which increased in size through the summer. Generalized additive models suggested M. leidyi abundance was significantly related to temperature, but not to salinity or depth. The Mar Menor lagoon is an anthropogenically-disturbed habitat that may favour this species. Blooms of M. leidyi in the Mar Menor lagoon are of great concern given its negative impacts in previously invaded habitats.