Within the next few years, many types of transgenic rice ( Oryza sativa ) will be ready for commercialization, including varieties with higher yields, greater tolerance of biotic and abiotic stresses, resistance to herbicides, improved nutritional quality, and novel pharmaceutical proteins. Although rice is primarily self-pollinating, its transgenes are expected to disperse to nearby weedy and wild relatives through pollen-mediated gene flow. Sexually compatible Oryza species often co-occur with the crop, especially in tropical countries, but little is known about how quickly fitness-enhancing transgenes will accumulate in these populations and whether this process will have any unwanted environmental consequences. For example, weedy rice could become much more difficult to manage if it acquires herbicide resistance, produces more seeds, or occurs in a wider range of habitats because of the spread of certain transgenes. Rice-growing countries urgently need publicly available ecological assessments of the risks and benefits of transgenic rice before new varieties are released.