The invention provides nucleic acids, and variants and fragments thereof, obtained from strains of Bacillus thuringiensis encoding .delta.-endotoxins having pesticidal activity against insect pests. The invention further provides mutagenized nucleic acids that have been modified to encode pesticidal polypeptides such as endotoxins having improved pesticidal activity and/or altered pest specificity. Particular embodiments of the invention provide isolated nucleic acids encoding pesticidal proteins that may be optimized as well as pesticidal compositions, expression cassettes, and transformed microorganisms and plants comprising a nucleic acid of the invention. These compositions find use in methods for controlling pests, especially plant pests.