The boundary effect on the drag force acting on a spherical floe having a nonhomogeneous structure is examined by considering a spherical floe at the centerline of a cylindrical tube. The floe is simulated by an entity having a two-layer structure, and its porous nature mimicked by varying the relative magnitude of the permeability of its inner and outer layers. The results of numerical simulation reveal that the tube wall has the effect of compressing the streamlines and vorticity contours. Also, as in the case of a rigid entity, the wake in the rear region of a floe, which arises from the convective motion of the fluid, is depressed. For fixed volume-averaged permeability, the influence of the tube wall on the behavior of a heterogeneous floe is more significant than that on the behavior of a homogeneous floe, and the influence varies with the structure of the former. The heterogeneous structure of a floc leads to a deviation in the modified drag coefficient-Reynolds number relation from a Stokes-law-like correlation. The smaller the average permeability of a floe the greater the deviation, but the presence of the tube wall has the effect of reducing the deviation. (C) 2003 Elsevier Inc. All rights reserved.