The present invention provides cyclopropane fatty acid synthase genes and proteins, and methods of their use. The present invention encompasses both native and recombinant wild-type forms of the synthase, as well as mutants and variant forms, some of which possess altered characteristics relative to the wild-type synthase. The present invention also provides methods of using cyclopropane fatty acid synthase genes and proteins, including in their expression in transgenic organisms and in the production of cyclopropane fatty acids in plant oils, and in particular seed oils.