Abstract. We consider the sufficiency of the Matkowsky condition concerning the differential equation ey" + f(x, e)y' + g(x, e)y = 0 (-a < x < b) under the assumption that/(0, e) = 0 identically in e, fx(0, e)=£0 with/ > 0 for x < 0 and/ < 0 for x > 0. Y. Sibuya proved that the Matkowsky condition implies resonance in the sense of N. Kopell if / and g are convergent power series for | e|< p (p > 0), /(x,0) = -2x and the interval [-a, b] is contained in a disc D with center at 0. The main problem in this work is to remove from Sibuya's result the assumption that D is a disc.