Strain relaxation and dislocation fort-nation for PbTe molecular beam epitaxy on 5.2% lattice-mismatched (001) PbSe substrates is studied using in situ reflection high-energy electron diffraction and scanning tunneling microscopy. For layers exceeding I monolayer (ML) in thickness misfit dislocations are formed along the fourfold in-plane (110) directions within the layer/substrate interface, These misfit dislocations are of pure edge type with a 1/2[110] type Burgers vector parallel to the interface. At layers exceeding about 5 ML, remarkably regular square arrays of misfit dislocations are formed with a dislocation spacing of 10 nm and a high uniformity of the spacings of +/-12%. This is due to the mutual lateral elastic repulsion between the dislocations as well as the fact that the (001) interface is a preferred dislocation glide plane in the lead salt compounds. (C) 2002 Elsevier Science B.V. All rights reserved.