Performance of solar water heaters in thermosyphonic flow with polypropylene and steel tubes was studied experimentally. An experimental apparatus consisting of 36 south-facing parallel tubes was designed and built especially for this study. Experiments were performed at Sharif University of Technology in Tehran during July-August 1994 from 0900 to 1700, when the ambient temperature varied from 29 to 36°C. Overall, 30 experiments were conducted for both types of tubes. At first, it was found that the best collector slope for both types of tubes was 36° and it is independent of tube type. Generally, it was found that polypropylene tubes under similar conditions can increase water temperature by 10°C more than steel tubes. Based on the results shown in this study, use of polypropylene tubes in solar water heating systems is recommended.