A rotating x‐ray camera is described which yields lattice constants of single crystals with a possible error of 0.00007kX.Measurements of expansion may be made with a precision of 0.00002kX.The design of the camera permits accurate specimen alignment at any specimen temperature and requires that few corrections be made. Lattice constants of pure Al, Au, CaF2, Cu, Ge, and Si at 25°C and of pure Ar and Ne at 4.3°K are reported and critically compared with values from the literature.