This paper presents a theoretical and experimental study of the shape of long isolated bubbles similar to those observed in horizontal slug flows. Bubbles of different volumes were studied, their lengths varying from 30 to 100 times the pipe diameter. Two different shapes were observed corresponding to plug and slug flow regimes. The transition between plug and slug flow was shown to depend on both Froude number and bubble length. A model based on the mass and momentum conservation equations of each phase is proposed. It is able to predict the transition between plug and slug flow regimes as well as the volume and the shape of long bubbles. It is shown that usual slug flow codes that aim to predict the void fraction tend to overestimate the gas fraction in the bubble region. The model may be easily implemented into slug tracking codes.