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A combined rat and mouse trapping grid was established in arid coastal forest on Santa Cruz Island, Galapagos over June and July 2009. Mouse traps were opened first and did not initially catch any mice. Rat traps were opened four days later and began catching rats on the second night. Mice were trapped in increasing numbers only after the rat catch-rate had declined substantially. Interference with bait at rat traps also increased. The estimated density of rats was 4.8 rats/ha and mice 32.3/ha. The results suggest mice were present but their activity and/or numbers were being suppressed by the larger rodent. This conclusion has implications for competitive exclusion of native Galapagos rodents by both introduced species. It also suggests caution for rodent abundance estimates and planned rodent eradications when two or more species are present.