A method and apparatus for positioning, aligning and orienting fibers in flowing fluids, to permit fibers such as wood pulp fibers to be accurately imaged and rapidly measured with little operator involvement, utilizing a plugging resistant and fouling resistant sheath flow cell, is described. An embodiment is disclosed which simultaneously determines the distributions of shape (curl) and length of samples of wood pulp fibers. Fibers are transported in a dilute water suspension through a sheath flow cell that orients fibers normal to an imaging system that includes a two-dimensional CCD video camera. Images of fibers are analyzed rapidly by a processor, which calculates the shape and length of individual fibers. The data are displayed on a cathode ray tube screen while a pulp sample is being analyzed, and are stored in processor memory for further analysis.