Abstract: A molecular or atomic beam source for use in molecular beam epitaxy comprises a hollow cylindrical crucible, preferably made of boron nitride, which is heated by an electric current passed through a plurality of very thin elongate metal strips disposed outside of and spaced apart from said crucible and mounted parallel to its axis. In order to maintain the strips in the correct position irrespective of temperature, they may be mounted on springs at each end, but preferably they are made in self supporting pairs linked at the open end of the crucible and mounted on brackets at the other end. The thin strips can be made self supporting by folding along lines parallel to their long axis in a variety of ways. They are arranged to present a large surface area to the crucible which ensures efficient heating and increases the maximum operating temperature of the source.