Ocean gravity waves measured at the Harvest Platform off the coast of California were analyzed using tools derived from methods of nonlinear dynamics. We compare these with laboratory data of random surface wave fields. The Harvest data show clear signs of low-dimensional dynamics in action and have an embedding dimension of six or seven. The largest Lyapunov exponent, a measure of future uncertainty, shows that the data are from a chaotic system and that predictions are limited to a horizon of about two major periods of the predominate wavelength. These results underscore the utility of nonlinear time domain methods for signal analysis and for extracting dynamical information not visible with linear methods.¿ 1997 American Geophysical Union