A vein-yellowing disease of Ageratum conyzoides in Singapore was shown to be caused by a geminivirus, here named ageratum yellow vein virus (AYVV), which was transmitted by the whitefly Bemisia tabaci but not by inoculation with sap or through seed. AYVV particles (30 ?? 20 nm) are serologically related to those of other whitefly-transmitted geminiviruses, and reacted with some monoclonal antibodies elicited by particles of African cassava mosaic or Indian cassava mosaic geminiviruses. However, the epitope profile of AYVV differed from the profiles of these viruses, and from those of geminiviruses from vein yellowing-affected A. conyzoides from India and from yellow leaf curl-affected tomato from either Singapore or India. The results provide further evidence of antigenic differences among geminiviruses that cause similar diseases in the same plant species in different geographical regions.