This article presents a method of adjusting the tempo of a music software sequencer so that it remains synchronized with a drummer's musical pulse. This allows music sequencer technology to be integrated into a band scenario without the compromise of using click tracks or triggering loops with a fixed tempo. Our design implements real-time mechanisms for both underlying tempo and phase adjustment using adaptable parameters that control its behavior. The aim is to create a system that responds to timing variations in the drummer's playing but is also stable during passages of syncopation and fills. We present an evaluation of the system using a stochastic drum machine that incorporates a level of noise in the underlying tempo and phase of the beat. We measure synchronization error between the output of the system and the underlying pulse of the drum machine and contrast this with other real-time beat trackers. The software, B-Keeper, has been released as a Max for Live device, available online at www.b-keeper.org .