Interplot competition in plant breeding experiments can substantially reduce the efficiency of plant selection. In diallel experiments on outcrossing species such as maize (ZeamaysL.), competing stresses might result from differences in vigour between inbred and hybrid entries. A partially balanced 8×8 triple lattice is presented. The design accommodates the inbred parents and F1single crosses of a 10×10 half diallel cross of maize in such a way that competing stresses are minimized. Practical field-plot considerations in the design are also discussed.