Two species of hydromedusae, assumed to be native to the Black and Caspian Seas, were routinely collected in Suisun Slough, California, at the Suisun City Marina, during late summer and fall of 1997. Suisun Slough connects directly with Suisun Bay, part of the biologically complex and commercially important upper San Francisco Estuary, and with San Francisco Bay via San Pablo Bay. Maeotias marginata (Modeer 1791), has been previously reported (as M. inexspectata) from the Petaluma River, another tributary entering San Pablo Bay, while an as-yet undetermined species of Moerisia represents a new distributional record for this genus in the eastern Pacific. Morphologies of the adults and immature growth stages of medusae of both species are described. The polyp stages of both species were reared in the laboratory following spawning of adult medusae collected from the field. Both species are apparently representative of the robust and aggressive Black and Caspian Seas brackish water fauna, many species of which have been introduced into estuarine habitats worldwide. The potential of these planktonic predators to alter zooplankton communities and feed directly on larval and juvenile stages of threatened native and commercially valuable estuary fish species are all possible, but remain uninvestigated.