The effect of pH on the urease activity of the extract from ileum-cecum contents of rats revealed the presence of an enzyme having hign activiy in the acid region (pH 2-3). This type of enzyme was designated as acid urease to distinguish it from the enzyme having high activity in the neutral region. The bacterium producing acid urease was isolated from the cecum contents of rats and identified as Lactobacillus fermentum. Acid urease was partially purified from the extract of L. fermentum TK1214. The enzyme had the optimal pH at 2.4, the isoelectric point at 4.2, and a K_m of 17 mM, and was inhibited by caprylohydroxamic acid, N-ethylmaleimide and p-chloromercuribenzoic acid, though these compounds were considerably less inhibitory to acid urease than to neutral urease.