A new species of naked amoeba, Platyamoeba pseudovannellida n.sp., is described on the basis of light microscopic and fine structural features. The amoeba was isolated from the Salton Sea, California, from water at a salinity of ca. 44‰. Locomotive amoebae occasionally had a spatulate outline and floating cells had radiating pseudopodia, sometimes with pointed tips. Both these features are reminiscent of the genus Vannella. However, the surface coat (glycocalyx) as revealed by TEM indicates that this is a species of Platyamoeba. Although salinity was not used as a diagnostic feature, this species was found to have remarkable tolerance to fluctuating salinity levels, even when changes were rapid. Amoebae survived over the range 0‰ to 150‰ salt and grew within the range 0‰ to138‰ salt. The generation time of cells averaged 29 h and was not markedly affected by salt concentration. This is longer than expected for an amoeba of this size and suggests a high energetic cost of coping with salinity changes. The morphology of cells changed with increasing salinity: at 0‰ cells were flattened and active and at the other extreme (138‰) amoebae were wrinkled and domed and cell movement was very slow. At the ultrastructural level, the cytoplasm of cells grown at high salinity (98‰) was considerably denser than that of cells reared at 0‰.