Optically thick metal films perforated with a periodic array of subwavelength holes show exceptional transmission properties. The zero-order transmission spectra exhibit well-defined maxima and minima of which the positions are determined by the geometry of the hole array. We show that the minima are the collection of loci for Wood’s anomaly, which occurs when a diffracted beam becomes tangent to the film, and that the maxima are the result of a resonant excitation of surface plasmons ~SP’s !. SP’s from both surfaces of the metal film are apparent in the dispersion diagram, independent of which side of the film is illuminated, indicating an anomalously strong coupling between the two sides. This leads to wavelength-selective transmission with efficiencies that are about 1000 times higher than that expected for subwavelength holes. @S0163-1829~98!06332-2# On the surface of a metal, collective excitations of the electron density lead to the formation of surface plasmon ~SP! polaritons. 1 Light in the visible to near-infrared range does not couple to surface plasmons on a smooth metal-air interface; however, a periodic structure allows the optical probing of the surface plasmon dispersion, in a well-defined Brillouin zone. SP polaritons have in general been observed in reflection mode on metallized gratings, appearing as an absorption feature in the intensity of a diffracted order, allowing the study of the band structure. 2‐8 Recently, we reported on the highly unusual transmission properties of metal films perforated with a periodic array of subwavelength holes. 9 In these samples the interaction of the incident radiation with the surface plasmons leads to an enhancement of the transmission. The zero-order transmission spectra are characterized by well-defined maxima and minima of which the positions are determined by the geometry of the hole array. Most surprisingly, we have observed transmission at wavelengths up to ten times the hole diameter with efficiencies that can exceed unity when normalized to the area of the holes, where standard aperture theory 10 predicts a transmission efficiency of order 10 23 . In this article we analyze the zero-order transmission spectra in a significant fraction of the Brillouin zone. We have identified the minima as being due to Wood’s anomaly, 11 which is observed in diffraction gratings when a diffracted order becomes tangent to the plane of the grating. We further show that the transmission maxima are the result of a resonant excitation of surface plasmons. Moreover, we demonstrate that the surface plasmons on both sides of the metal film are excited equally strongly by the incident light. This implies that the very high transmission efficiencies observed are related to an anomalously strong coupling of the SP modes on both sides of the metal film through the holes. The samples are optically thick Ag films of thickness 0.2