The virtual mass of a long, transversely vibrating tube submerged in a liquid was experimentally determined. The tube was surrounded by rigid tubes in a hexangular array and in square array, at pitch-to-diameter ratios from 1.25 to 1.5. The inertia coefficients observed were considerably larger than for a cylinder in an infinite volume of fluid, but smaller than one would compute assuming an annulus inscribed within the clear space around the vibrating tube. The results show the reduction of natural frequencies in a submerged tube bundle for one particular relative motion of tubes. Experimental results concerning the increase in damping due to the surrounding tubes are briefly indicated.