From June 1975 to May 1991 we collected samples of Oniscidea (Isopoda) from 197 sites in the area surrounding San Francisco Bay, California, including the outer coast from Bolinas Lagoon, Marin County to Half Moon Bay, San Mateo County. Seventeen species of isopods were represented in these collections. [Nine of the species were entirely restricted to littoral habitats: Ligia occidentalis Dana, 1853; Ligia pallasii Brandt, 1833; Littorophiloscia richardsonae (Holmes and Gay, 1909); Mauritaniscus littorinus (Miller, 1936); Detonella papillicornis (Richardson, 1904); Armadilloniscus coronacapitalis (Menzies, 1950); Armadilloniscus lindahli (Richardson, 1905); Armadilloniscus holmesi (Arcangeli, 1933); and Alloniscus perconvexus (Dana, 1856). Two species were riparian: Ligidium latum (Jackson, 1923) and Ligidium gracile (Dana, 1856). The remaining six species were found in a variety of terrestrial habitats: Protrichoniscus heroldi (Arcangeli, 1932); Porcellio scaber (Latreille, 1804); Porcellio dilatatus (Brandt and Ratzeburg, 1833); Porcellio laevis (Latreille, 1804); Porcellionides floria (Garthwaite and Sassaman, 1985); and Armadillidium vulgare (Latreille, 1804).] Two of the species collected have not been previously reported from San Francisco Bay. All 19 species known or expected in the San Francisco Bay Area are discussed and figured, and a key and information concerning their distributions both within the Bay Area and throughout North America are provided. The results of this survey are discussed in relation to previous reports, and several errors in the literature concerning the oniscid isopods of the San Francisco Bay Area are corrected.