Data are reported on marine mammal mortality collected from California’s gillnet fisheries for California halibut and Pacific angel shark (set net) and swordfish and pelagic sharks (driftnet) during the first six months of a three-year mortality assessment program. NOAA Fisheries (NMFS) observer-technicians collected information on species composition, mortality and life history data from entangled cetaceans, pinnipeds and sea otters. Four harbor porpoises, three southern sea otters, 13 elephant seals, 30 harbor seals, 67 California sea lions, one unidentified otariid, and two unidentified pinnipeds were observed killed in the set net fisheries. Estimated total incidental mortality for these species during the six month period was: harbor porpoises (central California), 44 (SE=25); southern sea otters (central California), 33 (SE=18); elephant seals (central California), 144 (SE=58); harbor seals, 392 (SE=83); and California sea lions, 847 (SE=134). Nine common dolphins, three Pacific white-sided dolphins, one short-finned pilot whale, one Dall’s porpoise, one mesoplodont beaked whale, four elephant seals, one harbor seal, two California sea lions and two unidentified otariids were observed killed in the driftnet fishery. Estimated total incidental mortality for these species during the six month period was: common dolphins, 203 (SE=82); Pacific white-sided dolphins, 68 (SE=38); short-finned pilot whales, 23 (SE=22); mesoplodont beaked whales, 23 (SE=22); Dall’s porpoise, 23 (SE=22); elephant seals, 90 (SE=43); harbor seals, 23 (SE=22); and California sea lions, 90 (SE=62). Biological data including total length, sex, reproductive condition and age were collected from a subset of animals. Sex and length data were collected for some animals; age and reproductive condition have not been determined for most.