We present experimental results and their interpretation on the propagation of surface acoustic waves on a quasiperiodically corrugated solid. The surface is made of a thousand grooves engraved according to a Fibonacci sequence. For the first time, we observe the spatial structure of the critical proper modes obtained from an optical diffraction experiment. These special modes are characteristic of quasiperiodic systems and exhibit remarkable scaling features. PACS numbers: 68.35.Gy, 03.40.Kf, 71.55.Jv It is usually accepted that wave propagation in disordered structures leads to the phenomenon of Anderson localization 1 at any nonvanishing disorder for space dimensions rf=l and 2 and at sufficiently high disorder for rf = 3. The localization regime is a nonperturbative effect involving coherent interferences between all the wavelets partially reflected by the quenched disordered set of scatterers, for which only partial scenarios exist. According to the present wisdom, wave propagation in random media is characterized by the existence of a remarkable localization transition separating an "extended regime" at small disorder and a "localized regime" at large disorder, only at space dimension d > 2. At lower dimensions, any nonvanishing disorder leads to the second regime. The existence of a transition between two regimes is always exciting because one can hope that understanding the crucial features which trigger the transition will reveal the physics of the different regimes.In this respect, wave propagation in one-dimensional id = 1) quasiperiodic systems is very interesting since it has been discovered 2 that there exists a transition between an extended and a localized regime similarly to what occurs in d = 3 disordered systems.' d = 1 quasiperiodic systems are thus natural intermediate cases between periodicity and randomness. Another motivation for studying these systems follows from the recent experimental discovery of the quasicrystal phase in metallic alloys. 3 From a different point of view, since we have studied this problem in the context of surface acoustic waves (SAW's), the understanding of the interaction of elastic surface acoustic waves with complex surface topography 4,5 is of major importance to underwater acoustics, seismology, surface-acoustic-wave devices, nondestructive testing, and ultrasonic applications in medicine.In this Letter, we report experimental measurements on the spectrum and the proper modes of a Rayleigh surface acoustic wave propagating on the quasiperiodically corrugated surface of a piezoelectric lithium niobate (KZ-LiNbCh) substrate with a thousand grooves engraved according to a Fibonacci sequence. It is shown that this system is in the critical regime of the localization transition predicted to occur in certain quasiperiodic systems. With our choice of studying surface waves, an optical diffraction experiment using the Raman-Nath effect 6 allows us to probe for the first time the spatial structure of the proper modes with a remarkable precision, and to obtain informa...