This year, 30 million acres of the corn, cotton, and potatoes planted in the United States will have been genetically engineered to produce an endotoxin normally found in the microbe Bacillus thuringiensis (Bt), a self-contained pesticide that will be toxic only to target insects. Transgenic pest-resistant crops are a cost-effective alternative to chemical pesticides, and may offer a way to help feed the world's growing population with minimal environmental impact.