Heated and ambient temperature experimental ponds were used to examine competition between introduced pumpkinseed Lepomis gibbosus and native perch Perca fluviatilis in England, U.K., and how these interactions are likely to change under climate warming conditions. Results from three sets of two-week experiments indicated that in both species and in all sets, growth was faster in heated than in ambient temperature ponds. Growth of both P. fluviatilis and L. gibbosus in sympatry did not differ significantly from that observed in allopatric ponds. Diet analysis indicated that increased resource partitioning occurred when P. fluviatilis and L. gibbosus were reared in sympatry, with P. fluviatilis shifting to a diet higher in microcrustaceans. The results do not support the previous claims of adverse effects of L. gibbosus on P. fluviatilis populations. Under conditions of climate change, however, which have been demonstrated experimentally to enhance L. gibbosus recruitment, this species is expected to become invasive in England, resulting in higher densities that may exert a stronger competitive effect than examined in this study.