A rheological constitutive model is proposed for concentrated suspensions of bubbles. The proposed model is a modification of the Oldroyd constitutive model for the rheology of dilute emulsions of two immiscible Newtonian fluids. The proposed model predicts relative viscosity, reduced first normal stress difference, and reduced second normal stress difference, of concentrated suspensions of bubbles as functions of capillary number and bubble volume fraction. The model is evaluated using the available experimental data on the relative viscosity of bubbly suspensions. Further experimental studies are needed to generate data on normal stress differences in bubbly suspensions and to test the model predictions.