To decrease irreversible thermoinactivation of Aspergillus awamori glucoamylase, five Gly residues causing helix flexibility were replaced with Ala residues. Mutation of Gly57 did not affect thermostability. Mutation of Gly137 doubled it at pHs 3.5 and 4.5 but barely changed it at pH 5.5. The Gly139→Ala mutation did not change thermostability at pH 3.5, improved it at pH 4.5 and worsened it at pH 5.5. The Gly137/Gly139→Ala/Ala mutation gave 1.5–2-fold increased thermostabilities at pHs 3.5–5.5. Mutations of Gly251 and Gly383 decreased it at all pHs. Gly137→Ala and Gly137/Gly139→Ala/Ala glucoamylases are the most stable yet produced by mutation. Guanidine treatment at pH 4.5 decreased the reversible stabilities of Gly137→Ala, Gly139→Ala and Gly137/Gly139→Ala/Ala glucoamylases at infinite dilution while not changing those of Gly251→Ala and Gly 383→Ala glucoamylases, which is, in general, opposite to what occurred with thermoinactivation. Mutation of Gly57 greatly improved the extracellular glucoamylase production by yeast, that of Gly137 barely affected it and those of Gly139 and of both Gly137 and Gly139 strongly impeded it. These observations suggest that α-helix rigidity can affect reversible and irreversible glucoamylase stability differently, that the effects of multiple mutations within one α-helix to improve stability are not always additive and that even single mutations can strongly affect extracellular enzyme production.