The desire to use and control photons in a manner analogous to the control of electrons in solids has inspired great interest in such topics as the localization of light, microcavity quantum electrodynamics and near-field optics(1-6). A fundamental constraint in manipulating light is the extremely low transmittivity of apertures smaller than the wavelength of the incident photon, While exploring the optical properties of submicrometre cylindrical cavities in metallic films, we have found that arrays of such holes display highly unusual zero-order transmission spectra (where the incident and detected light are collinear) at wavelengths larger than the array period, beyond which no diffraction occurs, In particular, sharp peaks in transmission are observed at wavelengths as large as ten times the diameter of the cylinders. At these maxima the transmission efficiency can exceed unity (when normalized to the area of the holes), which is orders of magnitude greater than predicted by standard aperture theory, Our experiments provide evidence that these unusual optical properties are due to the coupling of light with plasmons-electronic excitations-on the surface of the periodically patterned metal film, Measurements of transmission as a function of the incident light angle result in a photonic band diagram, These findings may find application in novel photonic devices.