We have observed a systematic nucleation of misfit dislocations at the InGaN/GaN heterointerface. This occurs when InGaN films are grown on an epitaxially laterally overgrown GaN substrate with a reduced dislocation density. The misfit dislocations are aligned along 〈1100〉 directions forming a symmetric hexagonal array. Potential wurtzite slip systems were analysed by extending the Matthews-Blakeslee model to include Peierls forces. Due to an inactive basal plane in the c-growth direction, non-basal slip is necessary for plastic relaxation. The active slip system was identified to be {1122} 〈1123〉. The possibility of activation of other slip systems is also discussed. © 2003 WILEY-VCH Verlag GmbH & Co. KGaA.