A mechanism for the generation of spiral waves in three-dimensional ex
citable media is described. Experiments using the photosensitive Belou
sov-Zhabotinsky reaction shaw that circular or spiral waves appear in
the wake of a propagating wave following a local perturbation in illum
ination intensity. The induced waves arise from three-dimensional scro
ll waves striking the upper boundary of the excitable medium. Spiral w
aves,ur. formed over a range of illumination intensities and perturbat
ion times which place the induced wave in the vulnerable region of the
refractory wave back.