This is an account of nineteenth-century efforts to naturalize alien freshwater and anadromous fish in California. Between 1871 and 1896, government and private groups imported and released twenty-one species. Of these, fourteen became established, and at least seven are still important sport or food fish (see Table 1). No other state made a comparable effort during this period. The counterflow of native fishes to other regions was much smaller; only two Pacific slope species, the Chinook salmon (Oncorhynchus tshawytscha) and the rainbow trout (Salmogairdneri), were transported out of their native range before 1900. This one-sided exchange of fishes and the attendant transformation of fisheries was one aspect of a worldwide process that peaked in the nineteenth century, one that Alfred Crosby described as biological imperialism, in which European biota, both domestic and wild, colonized the temperate world.' As illustrated in this study, a variant of this movement took place during the internal colonization of North America. Only two European species came to California, the carp (cyprinios carpio) and the brown trout (Salmo trutta). The rest of the migrants were native to the eastern United States. Like Europeans, Americans were eager to naturalize familiar species in their homelands, but less enthusiastic about the movement of new plants and animals back to their old homes. Likewise, but less visibly, California's aquatic ecosystems have undergone modification by invaders just as much as the state's grasslands. Unlike plants, however, fish do not often migrate between watersheds without conscious human assistance. While it is possible to establish the time when certain plant species entered California, the process was inadvertent, not a result of human intention, and certainly not attributable to any individual. The introduction of exotic fish, by contrast, depended on conscious human effort and on a perceived need for such effort. Time, place, source, and agents of change are thus clear in the historical record. This article recounts both the institutional forces and the particular individual