A water tunnel study was conducted on a parallel triangular array of tubes with a pitch ratio of 1.375. The array was geometrically identical to that used previously in a wind tunnel study so that the tube response to cross flow could be compared. It was seen that the response curves for tube arrays in water are much less regular than those in air, creating ambiguity in defining the stability threshold. The irregularities are seen to be associated with shifts in relative tube mode and frequency. Arrays in water apparently first become unstable in one of the lowest frequencies of the band of frequencies associated with a given structural mode. The added mass coefficient computed from the observed frequency at instability is a little larger than the largest added mass coefficient obtained from existing theory for tube arrays in quiescent fluid.