In 2005, we discovered some plants of a zucchini yellow mosaic virus (ZYMV)-tolerant oilpumpkin breeding line (Cucurbita pepo), severely affected by a virus. Our first assumption was that ZYMV might have overcome the resistance, but ELISA tests revealed that most likely cucumber mosaic virus (CMV) caused the symptoms, although ZYMV was detected in very low concentrations as well. Using fruit flesh of such infected plants for inoculation of pumpkin seedlings lead to immediate death of all plants, independent of whether the plants were ZYMV-tolerant or susceptible. Therefore, we decided to isolate the CMV for further investigation.