Zucchini squash (Cucurbita pepo cv. Black Beauty) plants infected with A strain of Zucchini yellow mosaic virus (ZYMV-A) isolated from a hollyhock plant showed systemically severe mosaic symptom, similar to previously established Cu strain of ZYMV. However, initial symptom of squash infected by ZYMV-A strain was generally more severe than those infected by ZYMV-Cu. Using leaf-detachment assay, examination of kinetics of accumulation of the coat protein (CP) in systemic loaves of squash plants showed that CPs of ZYMV-A appeared earlier than those of ZYMV-Cu. However, both ZYMV-A and ZYMV-Cu showed similar kinetics of CP accumulation 7 days post-inoculation. These results indicate that different rates and initial severity of systemic symptom development were due to differences in the rate of movement rather than vims replication.