Time-resolved measurements of the temperature field in an agar-based tissue-mimicking phantom insonated with a large aperture 1-MHz focused acoustic transducer are reported. The acoustic pressure amplitude and insonation duration were varied. Above a critical threshold acoustic pressure, a large increase in the temperature rise during insonation was observed. Evidence for the hypothesis that cavitation bubble activity in the focal zone is the cause of enhanced heating is presented and discussed. Mechanisms for bubble-assisted heating are presented and modeled, and quantitative estimates for the thermal power generated by viscous dissipation and bubble acoustic radiation are given.