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Knowledge of the two-phase flow generated by breaking waves is desirable if we are to fully understand their effect on environmental processes such as air-sea gas exchange and production of sea-salt aerosol. However, due to the difficulties in making measurements in the high void fraction bubble plumes generated immediately after breaking, little detailed information is available. This paper describes laboratory measurements of the size of bubbles entrained in the dense plumes generated by wave breaking made using a pair of highly sensitive optical fiber phase detection probes. The results compare well with those of previous authors in the low void fraction parts of the flow, and go on to include data from within the highly aerated region present in the period shortly after breaking, close to the area of most active air entrainment. The data highlights the spatial and temporal evolution of the bubble sizes within breaker generated bubble plumes and demonstrates that some large air cavities with diameters of tens of millimeters are initially entrained. It is observed that the bubbles resident within the plume rapidly decrease in size with time and distance away from the point of primary entrainment as the large cavities initially entrained are broken up into smaller bubbles.