The geometrical distribution of the aqueous fluid in textural equilibrium with forsterite has been investigated by measurements of the dihedral angle, &thgr;, at pressures of 3 and 5 GPa and at 1000 ¿C. The measured &thgr; at 3 and 5 GPa are 48¿ and 40¿ to 42¿, respectively. Since a value of &thgr; smaller than 60¿ indicates that the aqueous fluid can form an interconnected network along the three-grain and through the four-grain junctions, the existing aqueous fluid in the mantle can migrate by percolation even at the small volume fractions in the pressure range. Combining these results with previously published results at lower pressure, it is suggested that the interconnected network of aqueous fluid is stable only at higher pressure than 2 GPa for the commonly accepted water content of the upper mantle. The present results show that the large scale transport of aqueous fluid is possible above 2 GPa, and that the physical properties of the upper mantle may change drastically at that pressure. ¿ 1998 American Geophysical Union