Blackfordia virginica (Mayer, 1910) is a small hydrozoan that has invaded estuaries around the world. In the lower Napa and Petaluma rivers, located within the San Francisco Estuary, B. virginica populations followed a classic pulsed bloom event. Medusae monthly bell diameter measurements showed an initial increasing trend with a wide range of sizes, followed by a decreasing average size and narrower size range towards bloom culmination. Biomass was generally greatest in June. Medusae were pelagic feeders, consuming both invertebrates and fish larvae. Copepod nauplii were by far most numerous prey in the guts, followed by cyclopoid copepods and mysids. Further studies of B. virginica are needed to understand its potential impacts on estuarine ecosystems, which may be great where it is abundant.