A transparent enlarged model of a six-hole injector used in the development of emerging gasoline direct-injection engines was manufactured with full optical access. The working fluid was water circulating through the injector nozzle under steady-state flow conditions at different flow rates, pressures and needle positions. Simultaneous matching of the Reynolds and cavitation numbers has allowed direct comparison between the cavitation regimes present in real-size and enlarged nozzles. The experimental results from the model injector, as part of a research programme into second-generation direct-injection spark-ignition engines, are presented and discussed. The main objective of this investigation was to characterise the cavitation process in the sac volume and nozzle holes under different operating conditions. This has been achieved by visualizing the nozzle cavitation structures in two planes simultaneously using two synchronised high-speed cameras.Imaging of the flow inside the injector nozzle identified the formation of three different types of cavitation as a function of the cavitation number, CN. The first is needle cavitation, formed randomly at low CN (0.5-0.7) in the vicinity of the needle, which penetrates into the opposite hole when it is fully developed. The second is the well known geometric cavitation originating at the entrance of the nozzle hole due to the local pressure drop induced by the nozzle inlet hole geometry with its onset at around CN=0.75. Finally, and at the same time as the onset of geometric cavitation, string type cavitation can be formed inside the nozzle sac and hole volume having a strong swirl component as a result of the large vortical flow structures present there; these become stronger with increasing CN. Its link with geometric cavitation creates a very complex two-phase flow structure in the nozzle holes which seems to be responsible for hole-to-hole and cycle-to-cycle spray variations.