How well does competitive theory explain the outcome in experimental markets? The authors examined the results of a large number of classroom trading experiments that used a pit-trading design found in Experiments with Economic Principles, an introductory economics textbook by Bergstrom and Miller. They compared experimental outcomes with predictions of competitive-equilibrium theory and with those of a simple profit-splitting theory. Neither theory was entirely successful in explaining the data, although in the first rounds of trading there was significant profit splitting and, as traders became more experienced, outcomes were closer to those predicted by competitive theory. (Contains 7 notes.)