To produce stable beams used by pencil beam interferometers, a new equal optical path beamsplitter (EBS) is developed. The beamsplitter divides one incident pencil beam into two parallel exiting beams with a fixed separation. The new EBS is a monolithic unit comprised of two right-angle trapezoidal prisms with a thin film beamsplitting coating on the faces, which are cemented together. Adjusting the dimensions of each element according to a simple algebraic relation produces two identical, parallel beams separated by a fixed distance with a zero optical path difference between them. The monolithic EBS is very simple and stable. It has applications in interferometers that require the use of a pencil beam source, such as the long trace profiler (LTP) and the precise angle monitor. If a large-diameter wavefront is incident on the EBS, it can be used in shearing interferometer applications, such as the lateral translation shearing interferometer and the rotational shearing interferometer. Use of the EBS in all of these configurations significantly improves instrument stability and accuracy. Some application examples and test results are presented.