The interaction of a swarm of bubbles with the isotropic grid turbulence generated in a vertical water channel has been studied experimentally. Measurements of local void fraction, mean velocity and all three turbulence intensity components are presented at a distance from the grid where isotropic turbulence prevails in single-phase flow. At low gas flow rate the interaction is rather mild with minor redistribution of the void but significant increase of the turbulence intensity. High gas flow rates result in significant modifications of the flow structure. Void distribution presents maxima at locations between the center of the channel and the wall reminiscing of the Segre-Sielberberg effect. Similar maxima are observed in mean velocity measurements at values much higher than the corresponding single-phase flow. Turbulence intensity components increase is following different trends in longitudinal and transverse direction and the isotropy of the flow is destroyed.