In surgical applications of lasers, it is often necessary to know when a laser has penetrated a bone. There are many instances when it is critical to avoid damaging tissue beneath the bone. We are developing a system to monitor the ablation of bone. We have found a method to detect when the bone has been penetrated by measuring the photo acoustic signal generated by a pulsed laser. Using a transducer on samples of temporal bone and several model substances, we can see a decrease in the power spectrum near 350 kHz as softer materials is ablated. The current results are from a carbon dioxide laser operating in the super pulse mode. We are developing the technique for use with the Vanderbilt Free Electron Laser as part of our computer assisted surgery techniques program.