Crystal conversion of beta-chitin to alpha-chitin by aq. NaOH treatment was studied for a highly crystalline beta-chitin sample from diatom spine. The minimum NaOH concentration to cause swelling was between 25% and 30% w/w. The alkali-swollen material was poorly crystalline and was regenerated as a-chitin on washing with water. This conversion caused total collapse of the original microfibrillar morphology. These features are similar to those of 7 N-8 N HCl treatment reported earlier, but alkali treatment was free from depolymerization or deacetylation.