The hop stunt viroid (HSV) has been purified from cucumber leaves infected with the hop stunt disease agent. Electron microscopic observation of denatured HSV shows a typical viroid RNA molecule of covalently closed circular form. The molecular length was estimated to be 290-300 nucleotides by electron microscopic measurement and polyacrylamide gel electrophoresis using potato spindle tuber viroid as a standard. HSV is one of the smallest viroids characterized so far. HSV preparations were separated into covalently closed circular and linear forms by polyacrylamide gel electrophoresis under fully denaturing conditions. Both circular and linear molecules were found to be infectious.