Pump–probe optical techniques with femtosecond laser sources can be used to provide information about the acoustical properties of very thin films. With this so-called ‘‘picosecond ultrasonics technique,’’ echoes shorter than 10 ps can be time-resolved in films whose thicknesses are larger than a few tens of nm; for smaller thicknesses, the acoustic excitation induces very high frequency acoustic resonances ranging from 0.1 up to 2.5 THz. In this work, the results of a picosecond ultrasonics experiment performed on gold colloids which represent an example of nanostructures is presented. The samples studied were of three types: solutions (gold particles dispersed in water), gels (gold particles embedded in a polymer matrix), and a film of particles attached on a polymer monolayer. The particle size was between 5 and 20 nanometers. In all the samples, oscillations in the transmitted probe beam which are ascribed to the vibrational mode of the sphere were observed. A second oscillation with a much longer period is also reported in the solution and gels cases. [Work supported by CNRS.]