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 degrees 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 degrees and it is independent of tube type. Generally, it was found that polypropylene tubes under similar conditions can increase water temperature by 10 degrees C more than steel tubes. Based on the results shown in this study, use of polypropylene tubes in solar water heating systems is recommended.