To obtain an estimate of the learning ability of opossums, five tame ones were trained on a series of position reversals to a criterion. With 4 trials per day there was no consistent improvement over a series of 15 reversals. When each daily session was increased to as many trials as needed to reach criterion, errors dropped sharply, indicating the formation of a position learning set. Upon return to 4 trials per day, 3 of 4 animals continued to perform with few errors, showing transfer of learning set. Opossums do more poorly than rats on this task, which is in agreement with the relative phyletic level of the two species.
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