A method of preparing fertile transgenic Zea mays (corn) plants which stably express DNA encoding a Bacillus thuringiensis endotoxin, so as to impart insect resistance to the fertile transgenic plants is taught. The method comprises bombarding regenerable corn cells with DNA-coated microprojectiles, wherein the DNA is adjusted to be more efficiently expressed in corn than the native B. thuringiensis sequence, and regenerating a fertile transgenic plants from the transformed cells.