We estimate the physical supply potential of biofuels from domestic municipal solid waste, forestry residues, crops residues and energy crops grown on existing cropland using optimistic assumptions about near-term conversion technologies. It is technically feasible to produce a significant amount of liquid biofuel (equivalent to 30-100% of 2003 gasoline demand) without reducing domestically produced food and fiber crops or reducing the total calories available as domestic animal feed. Most of this supply can be attributed to the potential of energy crops, with the combination of municipal solid waste and forestry residues supplying between 10% and 30% of recent gasoline demand. Our modeling approach to energy crops is unique in that it explicitly models interactions between the feed and fuel system using an optimization procedure that adjusts cropland allocation among major crops subject to a simple food security constraint. Our modeling indicates that sizable increases in biofuel production need not result in decreased availability of food or animal feed, but will require changes in the composition of livestock diets away from hay and soymeal toward either whole corn or feed coproducts of biofuel processing such as distillers grains. Whole corn yields very high levels of digestible calories per land area, so shifting away from soymeal and hay to corn feed permits the same total level of digestible calories to be produced from a smaller area. Furthermore, the coproduction of animal feeds with biofuels relaxes the need to grow dedicated feed crops at all. Thus, under our food security constraint, energy crops which yield feed coproducts (such as corn ethanol) can be grown on a larger area than other energy crops, potentially yielding higher total levels of biofuel than other crops (such as switchgrass) that yield more biofuel but less animal feed per land area. When the food security constraint is lifted nearly 200% of recent gasoline demand could be met by liquid biofuels, corresponding to a scenario in which all current cropland is converted to high-yielding switchgrass. The size of our supply estimates indicate that while domestic biofuels can play a large role in transportation, achieving such high levels of ethanol production may not be socially or ecologically desirable, or may be extremely costly with costs expressed through higher food prices, biodiversity loss, water degradation, and soil erosion. Policies designed to protect natural resources and stabilize food prices should be implemented early in order to achieve a reasonable level of biofuel production that avoids pushing these boundaries.