DNA transfection is the introduction of molecular constructs into parasites in a manner permitting the expression of encoded genes. Transfection is an exceedingly powerful technique for analyzing gen etic regulatory elements and gene function, and is widely used in molecular biology. Transfection by electroporation has now been accom plished in five trypanosomatid genera. We describe protocols used in our laboratories for transfection of Trypanosoma brucei and Leish mania. There are three basic steps in DNA transfection: