Sodium alginate, mannuronic acid-rich and guluronic acid-rich fractions were prepared from "Mekabu" (sporophyll of Undaria pinnatifida). The production of short-chain fatty acids such as acetic, propionic and n-butyric acids from these fractions in mini-scale batch culture using pig cecal bacteria was studied, and the gas released from the culture was monitored. The volume of released gas corrected for blank value decreased in the order: glucose (a reference substrate) > guluronic acid-rich fraction > sodium alginate = mannuronic acid-rich fraction. The amounts of short-chain fatty acids produced from three fractions of alginic acid were smaller than that of glucose. These results suggested that alginic acid was poorly fermentable for hindgut bacteria and that its contribution to the host energy pool via microbial metabolism is small.