1. 1. Conjugation of o-aminophenol has been demonstrated in intact and homogenized hepatic caecum and fat body of Periplaneta americana. 2. 2. In homogenates, formation of the conjugate required the presence of uridine diphosphate glucose. Various glucosides, uridine diphosphate glucuronic acid and uridine diphosphate N-acetylglucosamine were without effect. 3. 3. The conjugate was hydrolysed by β-glucosidase preparations free of α-glucosidase activity; gluconolactone, an inhibitor of β-glucosidase, prevented this hydrolysis. Preparations of β-glucuronidase did not hydrolyse the conjugate. 4. 4. Attempts to demonstrate o-aminophenyl glucoside formation with β-glucosidase were unsuccessful. 5. 5. From these and other observations it is concluded that o-aminophenyl- β-D-glucoside may be synthesized in certain Periplaneta tissued by transglucosylation from uridine diphosphate glucose by a uridine diphosphate glucose glucosyltransferase. The enzyme uridine diphosphate glucuronate glucuronyl-transferase (E.C. 2.4.1.17) is absent from these tissues. 6. 6. Evidence is presented for the occurrence of uridine diphosphate glucose in Periplaneta fat body and for its formation there from uridine triphosphate and glucose-1-phosphate by uridine diphosphate glucose pyrophosphorylase (E.C. 2.7.7.9). 7. 7. Some properties of the uridine diphosphate glucose glucosyltransferase from Periplaneta fat body and hepatic caecum are described, and the effect of various sugars and nucleotides on o-aminophenyl glucoside synthesis by this enzyme is reported. 8. 8. These results are discussed in relation to the conjugation of phenols in various plant and animal tissues.