The results of cavitation tunnel and tank tests on an 800 mm diameter model of a marine current turbine (MCT) are presented. The tests were carried out in a 2.4 m×1.2 m cavitation tunnel and the 60 m towing tank. Results for power and thrust coefficients are presented for a range of tip speed ratio and pitch settings for various conditions. The results of this investigation provided an insight into the operation of a singe turbine in straight or yawed flow, the effect on performance of changes in the tip immersion of the rotor, the interference between twin rotors and the likely areas of cavitation inception. In addition, the analysed results presented provide useful information for the hydrodynamic design of MCTs and detailed data for the validation of numerical models.