Minced reproductive tissue has the highest capacity to synthesize trehalose in adult ascaris. Limited synthesis was obtained with muscle tissue but neither intestine nor perienteric fluid synthesize this disaccharide. Synthesis occurs under aerobic or anaerobic conditions at pH 7.4 to 7.6. No synthesis could be demonstrated in cell-free preparations fortified with ATP, Mg++, UDPG, and glucose. Trehalase activity was only observed in gut tissue.