The broad spectrum non-selective herbicide ammonium-glufosinate, combined with genetically modified glufosinate-resistant rice cultivars (GM rice), may comprise a rice weed control system for red rice and other weeds that infest Brazilian rice fields. The objective of this study was to evaluate the potential for outcrossing and gene flow of GM rice to red rice. Natural outcrossing between adjacent field plots (1 m x 1 m) was measured during the 1999-2000 growing season in Brazil. The outcrossing rate of the GM rice as the female parent to red rice, as the pollen donor, was 0.22% for straw hull and 0.02% for the black hull red rice, respectively. In the reciprocal crosses, when GM rice was the pollen donor and red rice, the female parent, the outcrossing rate was 0.26 and 0.14%, respectively, for straw hull and black hull red rice. The results of this study suggest that, under field conditions, outcrossing of glufosinate-resistant rice to red rice can occur at a measurable level. The potential frequency of hybrids would be minimum under situations where weed control measures help prevent the red rice from flowering.