ABSTRACT: The microfossil collection in the University of California Museum of Paleontology (UCMP) in Berkeley is among the worldâ€™s largest. Its current holdings exceed 200,000 slides, 15,000 bulk samples, and two million washed residues. The UCMP database currently includes 10,191 type specimens of which 2,398 are primary types representing 10 microfossil groups. Although Cretaceous and Tertiary foraminifera from California and the south-central United States dominate the collection, it components are from all continents and oceans, range in age from Precambrian to Recent, and include specimens representing all 11 of the primary microfossil groups: foraminifera, ostracodes, calcareous nannoplankton, radiolarians, diatoms, silicoflagellates, conodonts, acritarchs, chitinozoans, dinocysts, and sporopollen. Also represented are chrysomonads, prasinophycean and coralline algae, ebriidians, and microplankton incertae sedis. The collection is dominated by the largest slide collection of fossil foraminifera from the West Coast of North America. Notable acquisitions in the past 25 years are the Loeblich and Tappan (UCLA) and Stanford University (LSJU) collections, and those donated by the regional divisions of Arco and Texaco. Adoption of these and other â€œorphanedâ€ collections by the UCMP ensures their preservation and convenient accessibility for future study. The UCMPcollection spans the history of micropaleontology on theWest Coast of the United States, which began at the end of the 19th Century with studies by non-local paleontologists. In early 1920s, when the oil industry realized the value of microfossils in subsurface correlation, micropaleontology blossomed in California (as well as Texas) as a career-worthy discipline and slide collections soon sprouted out of the necessity to archive data and to create taxonomic reference sets. The core of the UCMP collection comprises its native component and the adopted UCLA and LSJU collections, and accounts for the more than 50,000 slides, including more than 10,000 primary and secondary types. Approximately 40,000 of those slides, including all of the types, have been entered into the UCMP digital database, which is publically accessible online. The other 10,000 are UCLA glass slides (mostly of Paleozoic palynomorphs) that are currently being processed. There is an additional backlog of another 79,000 slides representing 19 collections from individual micropaleontologists and more than two million slides and sample residues from the West Coast divisions of Arco and Texaco. Although some progress has been made on those, it will be many years before their curation and database entry approaches completion.