Incubation of rat liver microsomes with GDP-[14C]mannose and phenyl phosphate for 30 min, followed by paper-chromatographic analysis, revealed the presence of a new radioactive compound that was isolated and characterized as phenyl beta-D-mannosyl phosphate. The enzyme catalyzing this reaction was very closely related to a dolichyl beta-D-mannosyl phosphate-synthesizing activity found in the same microsomes. The two activities were affected in a parallel fashion by mild heating and were found in the same proportions in whole microsomes, in rough-surfaced microsomes, and in smooth-surfaced mirosomes. Increasing the concentration of phenyl phosphate from 0 to 15 mM resulted in a progressive reduction in the rate of mannose incorporation into endogenous dolichyl phosphate. It is concluded that the two reactions are catalyzed by a single enzyme. The labeled product from phenyl phosphate was rapidly excluded from the microsomes, while the product from dolichyl phosphate was retained in the microsomes, suggesting that phenyl phosphate may act as a mannose-trapping agent, thereby inhibiting mannosyltransferase reactions involving dolichyl phosphate as a glycosyl carrier.