Four species of synthetic oligonucleotide probes for the diagnosis of hop stunt viroid (HSV) and citrus exocortis viroid (CEV) were devised. Probe HSV-1 detected all the members of HSV group, such as HSV-hop, HSV-grapevine, HSV-cucumber, HSV-citrus and a viroid-like RNA isolated from plum trees affected by plum dapple fruit disease. Probe HSV-2 discriminated HSV-grapevine from the other members of HSV group. HSV-hop and HSV-grapevine consist of the same numbers of nucleotides, with only one nucleotide exchange. It was also shown that the two viroids were indistinguishable by their biological and physicochemical properties. However, by using probe HSV-2, HSV-hop and HSV-grapevine were apparently differentiated. Probe CEV-1 detected all the members of potato spindle tuber viroid (PSTV) group, such as PSTV, CEV and chrysanthemum stunt viroid (CSV). Probe CEV-2 discriminated CEV from the other members of PSTV group. It is thus emphasized that synthetic oligonucleotide probes are useful for the diagnosis of viroids and their related strains. It was discussed that the method can be used for the diagnosis of viruses and their related strains.