The acidophilic, thermostable a-amylase of Bacillus caldovelox displays a unique end-product profile and action pattern on starch. Maltohexaose is preferentially produced, with a maximum yield of 40-44% (w/w) from 35% (w/v) starch and dextrins (DE 9 and DE 18). Maltohexaose, the initial product of 1% (w/v) starch and 35% (w/v) dextrin (DE 42) hydrolysis, is subsequently converted into maltopentaose with a maximum yield of 30% (w/w). This reaction does not involve glucose production. Substrates were hydrolysed from the non-reducing end by either a uni-or multimolecular mechanism, with no hydrolysis of maltohexaose or smaller sugars. The Km values for soluble starch and maltoheptaose were 4.68 mg/ml and 2.13 x 10 -2 M, respectively. ) and a mutant Klebsiella pneumon- iae (Monma et al. 1983) that produce maltohexaose in yields of 25-40% (w/w). However, with the exception of B. amyloliquefaciens, maltohexaose is only an initial hydrolysis product and is subsequently broken down to smaller saccharides. In this respect the a-amylase of B. caldovelox produces a unique and particularly high yield of maltohexaose from starch, 44% (w/w), and cannot hydrolyse maltohexaose . Studies have shown that this high yield of maltohexaose is due to the selective hydrolysis of only larger oligosaccharides and preferential cleavage of maltohexaose from the non-reducing end of the substrate, linked to the inability of the enzyme to hydrolyse any maltooligosaccharide of less than seven glucose units.