Two experiments are reported in which the interaction of traces in pigeon short-term memory was studied in a delayed matching-to-sample task by presenting 2 stimuli, S1 and S2, in succession, and then presenting them simultaneously for a choice response, with choice of S2 always being reinforced. The presentation of S1 significantly interfered with choice of S2, and the magnitude of this effect was shown to be dependent upon the fixed-ratio requirements placed upon S1 and S2 and upon the length of time separating S1 and S2. It was argued that the theory which best accounts for these findings is one which holds that traces of varying strengths are established independently of one another and compete with one another to determine which stimulus will be chosen at the time of test. The results of Experiment III ruled out an alternative interpretation of these data in terms of strength limitation theory.