A survey of the mammalian fauna of Siwa Oasis was carried out during a period extending from autumn 2008 to winter 2010, inclusive. Fourteen mammalian species were recorded in this survey. Four species were recorded from the Oasis for the first time, namely, the charming dipodil, Dipodillus amoenus; shaw's jird, Meriones shawi; the house rat, Rattus rattus; and the red fox, Vulpes vulpes. The house rat and the red fox undoubtedly invaded the Oasis as a result of the urban and agricultural development in this part of Egypt. On the other hand, eight of the mammalian species known to occur in Siwa were not recorded in this survey; some of them are vulnerable, threatened, or endangered. Morphometric comparison study carried out between Gerbillus gerbillus collected from Siwa and that collected from the Mediterranean Coastal Desert as well as a comparison between different color variants of Dipodillus campestris revealed the presence of some significant and highly significant differences between them.