The cross-flow turbine has attracted much attention as a source of hydropower generation for small and micro-systems, especially for low head establishments. Such turbines have a distinct advantage of lower initial and operating costs over other small scale turbines, but their efficiency is lower than others. Efficiency predictions of these turbines are generally based on the assumption that the entire how crosses from the first stage to the second stage of the turbine runner. In this study, interior guide tubes were designed and used inside the runner of a cross-flow turbine to collect and guide the crossing how towards the second stage of The runner. The interior guide tubes were designed on the basis of observed flow patterns inside the runner. Experimentally, three different types of tubes were tested. The laboratory tests were conducted to calculate the turbine efficiency with different gate openings of nozzle and different positions of interior guide tubes. Results of this work with and without interior tubes have been presented in this paper. When the experiments were done with and without interior guide tubes, it has been found that turbine efficiency with the interior guide tube decreased about 5 per cent. Copyright (C) 2000 John Wiley & Sons, Ltd.