The future implantation of second generation marine current turbine arrays depends on the understanding of the negative interaction effects that exist between turbines in close proximity. This is especially the case when the turbines are axially aligned one behind another in the flow. In order to highlight these interaction effects, experiments were performed in a flume tank on 3-bladed 1/30th scale prototypes of horizontal axis turbines. This work focuses on the interactions between two horizontal axis marine current turbines, axially aligned with the upstream flow. Thrust and power coefficients function of the rotation speed of the downstream device are presented. Besides, the wake of each turbine is characterised so as to explain their behaviour. A large range of inter-device distances is considered, as well as two upstream turbulence intensity conditions, namely 3% and 15%. This latter parameter deeply influences the behaviour of a marine current turbine and thus plays a preponderant role in the interactions effects between two devices. Indeed, this study points out that, for the considered turbine and blade geometry, higher ambient turbulence intensity rates (15%) reduce the wake effects, and thus allows a better compromise between inter-device spacing and individual performance. (C) 2014 Elsevier Ltd. All rights reserved.