A general computer simulation analysis of the coupling efficiency between surface emitting LEDs and optical fibers has been carried out using ray tracing techniques. Coupling efficiencies were evaluated as a function of both the fiber numerical aperture (N. A. ) and the ratio of the fiber core diameter d//F to the LED emission diameter d//E for a spherical lens positioned between the LED and fiber. Coupling efficiencies near the theoretical maximum values were obtained for d//F/d//E less than equivalent to 5 for 0. 20-N. A. fibers and for d//F/d//E less than equivalent to 2. 5 for 0. 40-N. A. fibers by optimizing the len's index of refraction and the LED-to-lens spacing. 24 refs.