Sensor data fusion has long been recognized as a means to improve target tracking. Common practice assumes that the sensors used an synchronous, i.e., they have identical data rate, measurements are taken at the same time, and have no communication delays between sensors platform and central processing center. Such assumptions are invalid in practice. This paper deals with removing such assumptions when considering the multi-sensor target tracking case. In particular, it assumes that the sensors used can have different data rates and communication delays between local and central platforms. A new target tracker using asynchronous sensors is proposed and derived in this paper. The performance of the filter is compared to the optical sequential filter using simulated targets.