AbstractThe behavior of rigid circular cylinders suspended vertically in viscous fluids in laminar shear flow was found to compare favorably with a theory for infinitely long cylinders which we have extended to include interactions between two cylinders of different diameters. A description of the versatile Couette apparatus in which most of the experiments were performed is included. Observations were made on single cylinders at various distances from the wall bounding flow, on collisions between cylinders of both equal and unequal diameters and on interactions between a cylinder and a sphere. Collision doublets were found to be either permanent or separating, depending on the initial orientations and separations of the particles. The behavior of these particles is analogous to that of spheres in Couette flow.