Abstract: We have observed N∶N-1(N≥2) frequency－locking of propagating wave fronts when the light intensity is increased in a spatially extended system. The experiments were carried out using the light－sensitive form of the Belousov-Zhabotinsky reaction. By constructing a mapping function, the characteristic devil′s staircase can be reproduced when plotting wave period versus light intensity, in agreement with the experimental data.