Following training on an easy size discrimination, pigeons were matched on the basis of stimulus control by the positive stimulus following generalization tests. Three subjects were immediately retrained on a more difficult discrimination along the same dimension, while the remaining three subjects were retrained after a six-month delay. The six-month delay group took longer to learn the more difficult discrimination and displayed less stimulus control on a subsequent generalization test. The intended audience consists of individuals interested in basic learning research and its application. (Author)