Creeping bentgrass (Agrostis stolonifera L.) is a commercially important turfgrass used principally on golf courses. Weed control is one of the major problems encountered in golf course maintenance, largely because the major weed, Poa annua L., is another grass species with herbicide responses similar to creeping bentgrass. Development of creeping bentgrass cultivars expressing one of the herbicide resistance genes would provide an effective solution. The prospects of commercialization of transgenic cultivars of creeping bentgrass have raised questions about the potential for pollen-mediated gene flow to related Agrostis spp. In a field study we have measured the frequency of interspecific hybridization between transgenic creeping bentgrass and four related species, A. canina L., A. castellana Boiss. and Reut., A. gigantea Roth, and A. capillaris L. Interspecific transgenic hybrids were recovered between creeping bentgrass and A. capillaris and A. castellana at frequencies of 0.044 and 0.0015%, respectively, which were considerably lower than intraspecific transgenic progeny recovery in the same experimental plots (0.631%). No interspecific transgenic hybrids were recovered with A. gigantea or A. canina.