Abstract: The highly nonlinear response of a bubble to an acoustic excitation makes nonlinear methods possible for bubble observation and sizing. There are various nonlinear acoustic methods of bubble detection. The difference frequency method is described, with application to the bubble concentration measurements in the sea. The results of observations on bubble populations of different sizes with a nonlinear acoustic bubble counter are given. The bubble counter was put in the subsurface ocean layer from the drifting vessel. The unit detected single bubbles of known sizes when they passed through the working volume created by the intersection of high-frequency acoustic beams. Data on bubble concentration are presented and discussed. Experiments on bubble motion diagnostics in the cavitation water jet are also described.