Many varieties of transgenic rice are under development in countries where wild and weedy relatives co-occur with the crop.  To evaluate possible risks associated with pollen-mediated transgene dispersal, we conducted a two-year survey in Vietnam to examine overlapping flowering periods of rice (Oryza sativa L.), weedy rice (O. sativa), and wild rice (O. rufipogon Griff.), all of which are inter-fertile.  We surveyed populations in two regions of the Mekong Delta, northern and southern, and at three sites in each of three habitats per region: fresh water, saline water, and acid sulfate soil.  Weedy rice frequently flowered simultaneously with neighboring cultivated rice plants.  Flowering was more seasonal in wild rice and often peaked in November and December.  Peak flowering times of wild rice overlapped with adjacent rice fields at all of the saline sites and half of the acid sulfate sites.  The longer flowering season of wild rice ensured that crop-to-wild gene flow was possible in fresh water habitats as well.  Our second objective was to determine whether wild and weedy rice populations are exposed to pests that could be targeted by future transgenes, which may then provide fitness benefits.  These populations shared many pathogen and insect herbivore species with cultivated rice (leaffolder, locust, cricket, planthoppers, rice bug, stem borer, sheath blight, blast, bacterial leaf blight, and brown spot).  Damage by leaffolders and locusts was the most frequently observed insect feeding damage on all three rice types.  Indicator species analysis revealed that most of the insect herbivores were associated with particular habitats, demonstrating the importance of broad geographic sampling for transgenic rice risk assessment.  These survey data and the strong likelihood of gene flow from cultivated rice suggest that further studies are needed to examine the effects of transgenic traits such as resistance to pests on the abundance of wild and weedy rice.