Experiments were conducted to study the properties of bubbly flows in elastic fluids with nearly constant viscosity (Boger-type fluids). The effect of gas volume fraction was investigated by injecting bubbles with a narrow size distribution in a vertical column filled with the test fluid. It was found that the dispersion of bubble changes dramatically depending on the bubble size: if the diameter of the bubbles is small, large vertical clusters are formed; on the other hand, the bubble assembly rises in a dispersed manner if the bubble size is increased. To understand the condition for which agglomeration occurs two additional experiments were conducted: the interaction of two side-by-side bubble chains was analyzed; and, the unsteady behavior of the first normal stress difference was studied in a rheometric flow. These analysis suggest that there is a process of accumulation of elastic stress. When the accumulated elastic stress surpasses the viscous repulsive stress, aggregation can occur. Interestingly, the critical diameter at which the bubble dispersion was observed to occur is close to that for which the velocity of an isolated bubble becomes discontinuous: the so-called bubble velocity discontinuity. This suggests that both phenomena share the same fundamental nature. We attributed the change of behavior to the modification of the gas-liquid interface. (C) 2012 Elsevier B.V. All rights reserved.