We have developed rectangular nanohole arrays in a gold film for enhanced sensitivity in biosensing applications. This paper describes the theory and fabrication of such arrays. Finite difference time domain simulations and experiments are used to determine the bulk sensitivity at 635 nm, and the sensitivity is improved by 14× for simulation and 7× for experiments with respect to circular hole arrays. Arrays with dimensions down to 20 nm are fabricated using focused ion beam milling. A stable vertical setup is built to image these arrays in a low noise environment. The results are encouraging for future biosensing tests.