The two Chrysanthemum viroids, Chrysanthemum stunt (CSV) and Chrysanthemum chlorotic mottle (CChMV) are detected by polyacrylamide gel electrophoresis ; the electrophoretic assay is an adaptation to slab gels (PASGE) of the Morris and Smith's procedure previously applied to Potato spindle tuber viroid. Use of ethidium bromide staining, specific of double stranded nucleic acids, makes the method much more sensitive than after toluidine blue 0 staining or by U.V. scanning of cylindrical gels.Compared to the biological indexing the technique appears highly sensitive : this biochemical test allows CSV detection in samples as small as 10 mg plant material.By using PASGE the presence of viroids can be noticed earlier after infection than with the normal biological indexing : viroid RNA can be observed in plants which do not show any symptom.The importance of sampling is emphasized. Viroids are detected in any part of the infected Chrysanthemum plants. Nevertheless, viroids concentration is higher in the tips and the young leaves which are the best material for indexing.This technique has been applied to a large number of samples. It is simple, rapid, highly sensitive and reliable and consequently seems to be very appropriate to a large scale indexing.