In the United States, several feral populations of African Clawed frogs (Xenopus laevis) have been documented. USGS has created a map that summarizes all these areas: http://nas.er.usgs.gov/queries/factsheet.aspx?SpeciesID=67

It is very possible that the frogs have set up populations beyond what is shown on this map, but because so few people are looking for these elusive animals, they are simply overlooked. It can be a challenge to find African Clawed frogs in the wild because they are fully aquatic and therefore spend their entire lives in the water, often hiding in vegetation. The frogs will migrate over land when forced to is situations where their water source dries out.
As far as I know, no one has performed a systematic search for these frogs in California. Typically, their populations are noted when someone stumbles upon them. I would like to complete a search for these frogs to determine where they have established populations in order to get an idea of the extent of damage they may be inflicting on California’s ecosystems.