We describe an insert for optical and magnetic‐resonance experiments on single molecules in a solid matrix at liquid‐helium temperatures. The experimental arrangement allowsinsituadjustment of the focusing lens and of the sample. A parabolic mirror serves to collect the fluorescence emission and to direct the light onto a photodetector. Microwaves can be irradiated through a coil around the sample while a superconducting magnet provides the possibility of a stationary magnetic field. ©1995 American Institute of Physics.