The standard picture of growth at a screw dislocation assumes that the movement of adatoms on a dislocation loop is the same as on an ideal plane. We have examined this proposition by investigating the-movement of a single tungsten adatom on a W(110) plane intersected by a screw dislocation. Surprisingly enough, adatom movement was entirely different than on a normal (110) plane: the overall diffusivity was higher, and the mobility varied with the location of the adatom relative to the dislocation core. This study demonstrates that surface transport is strongly affected in the vicinity of dislocations.