The extincion efficiency of transparent spheres with a radial gradient of refractive index irradiated by electromagnetic plane waves was obtained by using the finely stratified sphere model [L. Kae, P. Massoli, Appl. Opt. 33 (1994) 501-511]. The extinction efficiency of gradient-index microspheres and location changes of morphology-dependent resonances on the Mie extinction efficiency curves are discussed.