PREVIOUS work has shown that knockout mice lacking the fyn tyrosine kinase gene (fyn-/-) are impaired in spatial learning. Here, we have re-examined the spatial learning of fyn-/-mutants in an open field water maze. Unlike wild-type mice, fyn-/-knockouts often floated without moving when placed in the water but could swim adequately when their hind feet were mechanically stimulated. Under these conditions, fyn-/-mice showed significant improvement over trials in locating a hidden platform. On a transfer trial, at the end of training, they spent a disproportionate amount of time swimming in the location of the previously hidden platform. These findings suggest that fyn-/-knockouts are capable of spatial learning, but suffer an impairment that compromises their ability to swim normally.