Rana catesbeiana (American Bullfrog) is exotic in western North America and is sympatric with two native species, Rana luteiventris (Columbia Spotted Frog) and Pseudacris regilla (Pacific Treefrog). In a laboratory experiment, we assessed the growth of tadpoles of R. luteiventris and P. regilla in the presence and absence of tadpoles of R. catesbeiana. We found that in the presence of tadpoles of R. catesbeiana, P. regilla exhibited rapid growth in early larval stages compared to R. luteiventris. A second experiment indicated that, when reared with R. catesbeiana, P. regilla increased activity levels; this behavioral disparity may partly explain the observed difference in growth responses of the two frog species to R. catesbeiana competition. Our results suggest that tadpoles of P. regilla display greater growth and behavioral plasticity than do R. luteiventris, when subject to competition with R. catesbeiana. Rana luteiventris may be more susceptible than P. regilla to R. catesbeiana-mediated numerical decline, if exploitative or interference competition between tadpoles is a population-limiting factor in western breeding ponds.