We studied the Cladocera in four shallow brackish lakes at El Hondo and Salinas de Santa Pola in Mediterranean Spain from February 1999 to June 2000, sampling each month. We recorded five species : Daphnia magna, D. pulex, D. longispina, Simocephalus exspinosus and Moina brachiata. D. magna was the most abundant species and the only one present all year round. Total abundance of Cladocera varied significantly between wetlands and months, and was highest in the largest, deepest, least saline wetlands. For a given wetland and month, abundance increased at points of greater depth or lower salinity. Species richness of Cladocera was highest in those seasons and lakes where salinity was lowest, and in those seasons and lakes where cladoceran abundance was highest. These wetlands hold a particularly high number of waterbird species, but a very low number of Cladocera species. Ready dispersal via waterbirds does not appear to promote cladoceran diversity in this system, owing to local conditions such as high salinity. We suggest that the conservation value of wetlands for birds is not a good predictor of value for Cladocera.