Atomic force microscopy has been used to investigate the complexes formed between high molecular weight amylose chains and Aspergillus niger glucoamylase mutants (E400Q and W52F), wild-type A. niger starch binding domains (SBDS), and mutant SBDs (W563K and W590K) lacking either of the two starch binding sites. The images are interpreted in terms of a favourable binding between the amylose chains and the wild-type SBDs, leading to the formation of ring-like structures, in which parallel strands of the amylose molecule bind to both binding sites on the SBDs. The SBDs; are seen to form a template for the assembly of an expanded amylosic double helix. T his model for amylose-SBD binding has been used to propose a molecular mechanism for the role of the SBD in the hydrolytic action of glucoamylase on starch granules. The SBDs are considered to recognise the ends of amylosic double helices formed by the short amylosic chains present as branches on the amylopectin molecules, and displayed on the face of crystalline lamellae. This allows the binding and immobilisation, of chain ends by the SBD, facilitating binding and cleavage by the exo-acting catalytic domain.