Three different hypotheses for the occurrence of the brackish water bivalve Cerastoderma glaucum (Poiret, 1789), and the gastropod Potamides conicus (de Blainville, 1826) in the Bitter Lakes of Egypt are discussed and compared. The suggestions that forementioned species are relicts or Lessepsian migrants are rejected. Key words: Cardiidae, Potamididae, Cerastoderma, Potamides, brackish water, Bitter Lakes, Egypt.