A unique underwater optical instrument is described that measures the volume scattering function (VSF) in the retro (180 degree(s)) direction, i.e., (beta) ((pi) ). The instrument, referred to as Beta Pi, was designed and built at SRI International and was recently deployed in Monterey Bay. Spectral measurements of direct backscatter are achieved by imaging the backward scattered light from a laser beam with a charged-coupled device (CCD) camera. A large beam splitter, approximately 23 cm in diameter, is used to obtain the necessary monostatic geometry for imaging light scattered from the beam in the retro direction. With a 200 mm focal length, f/2 lens, measurements of volume scattering over the angular range from exactly 180 degree(s) to about 179 degree(s) are achieved. The angular resolution of the CCD/lens system is 0.0057 degree(s). The resolution of the scattering measurements, however, is determined by the divergence half-angle of the beam, which is about 0.02 degree(s). A fiber optic cable transmits the laser flux from the surface to a collimator in the underwater housing, so that almost any laser with a fiber optic coupler can be used as the source. An in-water calibration scheme is described and it is shown that the accuracy of the measured VSF at 180 degree(s) is only slightly less than the accuracy of the measured spectral beam attenuation coefficient. A measurement of the VSF from 179 degree(s) to 180 degree(s), made in shallow waters in Monterey Bay using a Nd:YAG (532 nm) laser, is shown.