Electrostatic effects which take place in group-m nitrides in their wurtzite crystallographic phase have important consequences on the optical properties of (Al,Ga)N/GaN multiple quantum wells. A low-temperature photoluminescence study shows that the behavior of the transition energy versus the barrier width is the opposite of that currently obtained for more usual m-V semiconductor compounds like arsenides: when decreasing the barrier thickness, a blueshift is observed. This behavior is attributed to a redistribution across the samples of the huge built-in electric field (several hundreds kV/cm) induced by the polarization difference between wells and barriers. The trend of the barrier-width dependence of the electric field is reproduced by using simple electrostatic arguments. However, the field magnitude is higher than that predicted laking account only piezoelectric effects. This result points out the role of the spontaneous polarization in wurtzite nitrides. [S0163-1829(99)13627-0].