Abstract: A fiber optic probe having a sealed, cylindrical housing closed at one end by a transparent window and at the opposite end by a wall through which a plurality of optical fibers extend toward the window. Adjacent the window the fibers are radially and circumferentially spaced about the axis of the housing and converge along lines which intersect one another at a common point on the housing axis that is adjacent or beyond the outer surface of the window. At least one of the fibers transmits light from a source through the window to illuminate a zone of a fluid sample, whereby particles present in such zone scatter light therefrom to the remaining fibers for transmission through the probe housing to light detecting and measuring apparatus.