Processes of bubble formation and spreading in breaking wind waves are experimentally investigated in a wind-wave tank. The distribution and movement of bubbles relative to the wave form are measured using photographic techniques. The main mechanism of bubble formation in these waves is intermittent bubble entrainment by an ordered convergent flow on the leading slope near the crest. This conclusion is supported by a simple experiment for modeling the bubble entrainment, where a small jet of water is injected into water whose surface is at rest: bubbles are entrained by the water jet. In the oceans, the mechanism under consideration will be important in understanding the breaking process of smaller scale waves superposed on dominant waves. A model experiment also shows that the existence of a stagnation point on a water surface (convergent flow) is not a sufficient condition for bubble entrainment. The downward force of the convergent flow must be greater than the restoring force of the surface tension at the stagnation point for bubble entrainment to occur.