Family 4 glycoside hydrolases (GH4) represent an unusual group of glucosidases with a requirement for NAD+, divalent metal cations and reducing conditions.  AglA from Thermotoga maritima is a typical GH4 enzyme, requiring NAD+, Mn2+  and strongly reducing conditions for activity. We have determined the crystal structure of the protein complexed with NAD+ and maltose refined at 1.9 Å resolution. The NAD+ is bound to a typical Rossman-fold NAD+-binding site and the nicotinamide moiety is localized close to the maltose substrate. Within the active site the conserved Cys174 and surrounding histidines are positioned to play a role in the hydrolysis reaction. The electron density maps indicate that Cys174 is oxidized to a sulfinic acid. Most likely, the strongly reducing conditions are necessary to reduce the oxidised cysteine side chain. Notably, the canonical set of catalytic acidic residues common to other glucosidases is not present in the active site. This, combined with a high structural homology to NAD-dependent dehydrogenases suggests an unusual and possibly unique mechanism of action for a glycoside-hydrolysing enzyme.