A new mode of misfit defect formation has been observed for the first time in high quality Si/Ge strained-layer superlattices grown by molecular beam epitaxy on Ge(001). In order to investigate the transition from coherent to incoherent growth we have studied a set of samples with a varying number of superlattice periods by transmission electron microscopy. High-resolution lattice imaging reveals that strain relaxation occurs through successive glide of 90° (a/6)<211> Shockley partial dislocations on adjacent {111} planes. The resulting microtwins represent the only relaxation mechanism we observed in the samples. Applied Physics Letters is copyrighted by The American Institute of Physics.