Abstract: Double infection by zucchini yellow mosaic virus (ZYMV) and cucumber mosaic virus (CMV) of the cucurbit host Cucurbita pepo (zucchini squash) cv. Eskandarani resulted in a synergistic effect where symptoms in doubly infected plants were more enhanced than plants singly infected by either virus. ELISA values revealed that the concentration of CMV in doubly infected zucchini squash plants is significantly higher than that of singly infected plants. However, ZYMV titer did not increase significantly in case of double infection compared to singly infected plants. Increase of CMV in doubly infected zucchini squash cv. Eskandarani was confirmed by local lesion assays and specific infectivity and it increased with increase in time post inoculation.