Eleven species of the genus Margarinotus Marseul (Histeridae: Histerinae: Histerini) are known to occur in California. One of these, Margarinotus merdarius (Hoffmann) is adventive, and two others, Margarinotus ephemeralis Caterino, new species and Margarinotus thomomysi Caterino, new species are described here for the first time. The first of these is closely related to Margarinotus remotus (LeConte), and the second to Margarinotus fenderi Wenzel. Both of the new species are restricted to the nests of burrowing rodents, Spermophilus Cuvier and Thomomys Wied-Neuwied, respectively, and help indicate that the arthropod fauna of such niches is still inadequately documented.