The translational velocities of elongated bubbles in continuous slug flow were measured for various flow rates, pipe inclinations and pipe diameters. Measurements were carried out by cross-correlating the output signals of consecutive optical fiber probes and by image processing technique. In addition, the velocities of single elongated bubbles in a stagnant and in a flowing liquid were measured by the same techniques. For all cases the measured velocities were compared to appropriate correlations. The measured velocities of single elongated bubbles were in all cases predicted quite well by the correlations while the velocities in continuous slug flow, for certain cases, were considerably underpredicted. This discrepancy is ascribed to the influence of the dispersed bubbles in the liquid slug region. A simplified model is proposed to calculate the translational velocity in continuous slug flow. The results of the model compare well with the measured translational velocities of elongated bubbles in continuous slug flow. (C) 2002 Elsevier Science Ltd. All rights reserved.