A pandeid hydrozoan new to California, Amphinema sp., was collected in 1998 as a hydroid living on the non-indigenous bryozoan, Watersipora "subtorquata", attached to floats in Bodega Harbor 80 km north of San Francisco Bay. The hydroid was cultured in the laboratory and medusae it released were raised to maturity. No species name could be assigned because although the hydroid colony structure and morphology of the polyp most closely resemble descriptions of Amphinema rugosum, the immature and adult medusae best resemble A. dinema. These two described species are known from widely-spaced locations worldwide including Europe (British Isles and the Mediterranean), New England, the Caribbean, east Africa, India, Japan and China, implying that they may transport easily between sites by man´s activities. Such wide-spread distributions of both species, coupled with the notable absence of Amphinema sp. from Bodega Harbor during a number of previous field surveys in the 1970´s, strongly intimates that Amphinema sp. has been introduced from elsewhere into Bodega Harbor during the past 25 years. Two additional species of Amphinema medusae present on the west coast of North America are discussed.