The capacities of the begomoviruses Bean dwarf mosaic virus (BDMV) and Bean golden yellow mosaic virus (BGYMV) to differeBean dwarf mosaic viru certain common bean (Phaseolus vulgaris) cultivars were used to identify viral determinants of the hypersensitive response (HR) and avirulence (avr) in BDMV. A series of hybrid DNA-B components, containing BDMV and BGYMV sequences, was constructed and coinoculated with BDMV DNA-A (BDMV-A) or BDMVA-green florescent protein into seedlings of cv. Topcrop (susceptible to BDMV and BGYMV) and the BDMV-resistant cvs. Othello and Black Turtle Soup T-39 (BTS). The BDMV avr determinant, in bean hypocotyl tissue, was mapped to the BDMV BV1 open reading frame and, most likely, to the BV1 protein. The BV1 also was identified as the determinant of the HR in Othello. However, the HR was not required for resistance in Othello nor was it associated with BDMV resistance in BTS. BDMV BV1, a nuclear shuttle protein that mediates viral DNA export from the nucleus, represents a new class of viral avr determinant. These results are discussed in terms of the relationship between the HR and resistance.