We have measured the photoluminescence of stacked layers of self-assembled InP quantum dots in magnetic fields up to 50 T. In the samples with three stacked layers of dots strong electronic coupling was clearly observed for layer separations of 2 and 4 nm. For a sample with only two stacked layers of dots (separated by 4 nm) no coupling was observed. We attribute this difference in behaviour to a model in which the coupling is via holes that are weakly bound in the GaInP layers between the dots. Since the doubly stacked layers have only one such layer, no coupling is possible. (C) 2000 Elsevier Science B.V. All rights reserved.