We have developed a complete lithographic process combining focused ion beam writing, dry etching, and molecular beam epitaxy forinsitupreparation of buried InP‐based microstructures. A focused ion beam is used to locally remove an ultrathin oxide imaging layer growninsituon the surface of InP. The pattern is transferred into the underlying semiconductor by free Cl2etching with the patterned oxide layer acting as an etch mask. After removal of the oxide mask, GaInAs/InP heterostructures with excellent morphology and high luminescence efficiency can be grown on the patterned substrate. The entire process of mask formation, lithography, and regrowth can be carried outinsiturepeatedly, and used for creating fully buried microstructures.