The flat interface nerve electrode (FINE) is an alternative to cylindrical nerve cuffs for functional electrical stimulation (FES). By elongating the nerve in cross section, the FINE places more stimulating contacts around the nerve, and moves central axons closer to the electrode surface. Previous experiments have demonstrated that the FINE can activate selectively each fascicle in the cat sciatic nerve, and modeling studies have indicated that it should be possible to selectively activate groups of axons within individual fascicles. This hypothesis is tested using a combination of experimental and modeling techniques. Pairs of contacts stimulating the same fascicle were tested for subfascicle level selectivity, defined as the fraction of fibers activated by one contact but not by the other. It was possible to achieve greater than 90% selectivity with the FINE, but there was considerable variation in the results. The modeling studies showed that the selectivity achievable with a given contact pair depended strongly on the relative locations of the electrode and fascicle. Therefore, reshaping the cross section of a nerve can provide selectivity at the subfascicular level, but the electrode design must be optimized to improve selectivity across different nerve geometries.