Solar collectors need to be tilted at the correct angle to maximize the performance of the system. In this paper, the annual solar fraction of the system (the fraction of energy that is supplied by solar energy) is used as an indicator to find the optimum inclination angles for a thermosyphon solar water heater installed in nor-them and southern parts of Jordan. Calculations are carried out using the powerful computer program TRNSYS (Transient System Simulation). The system is assumed to operate with a daily hot water load of 150 1 at 55degreesC flowing during the day according to the widely used Rand consumption profile. The results show that the optimum inclination angle for the maximum solar fraction is about phi+(0-->10degrees) for the northern region (represented by Amman) and about phi+(0-->20degrees) for the southern region (represented by the town of Aqaba). These values are greater than those for maximum solar radiation (which is commonly used as an indicator) at the top of the collector by about 5 to 8degrees. (C) 2002 Published by Elsevier Science Ltd.