Venous sinusoids in bone marrow are the site of a large‐scale traffic of cells between the extravascular hemopoietic compartment and the blood stream. The wall of the sinusoids consists solely of a basement membrane interposed between a layer of endothelial cells and an incomplete covering of adventitial cells. To examine its possible structural specialization, the basement membrane of bone marrow sinusoids has now been examined by high resolution electron microscopy of perfusion‐fixed mouse bone marrow. The basement membrane layer was discontinuous, consisting of irregular masses of amorphous material within a uniform 60‐nm‐wide space between apposing endothelial cells and adventitial cell processes. At maximal magnifications, the material was resolved as a random arrangement of components lacking the “cord network” formation seen in basement membranes elsewhere. Individual components exhibited distinctive ultrastructural features whose molecular identity has previously been established. By these morphological criteria, the basement membrane contained unusually abundant chondroitin sulfate proteoglycan (CSPG) revealed by 3‐nm‐wide “double tracks,” and moderate amounts of both laminin as dense irregular coils and type IV collagen as 1–1.5‐nm‐wide filaments, together with less conspicuous amounts of amyloid P forming pentagonal frames. In contrast, 4.5–5‐nm‐wide “double tracks” characteristic of heparan sulfate proteoglycan (HSPG) were absent. The findings demonstrate that, in comparison with “typical” basement membranes in other tissues, the bone marrow sinusoidal basement membrane is uniquely specialized in several respects. Its discontinuous nature, lack of network organization, and absence of HSPG, a molecule that normally helps to maintain membrane integrity, may facilitate disassembly and reassembly of basement membrane material in concert with movements of adventitial cell processes as maturing hemopoietic cells pass through the sinusoidal wall: the exceptionally large quantity of CSPG may represent a reservoir of CD44 receptor for use in hemopoiesis. Anat Rec 264:294–304, 2001. © 2001 Wiley‐Liss, Inc.