Analysis of a survey of the 1998-99 and 1999-2000 seasons for the same 100 smallholders in the Makhathini Flats region of KwaZulu-Natal shows that Bt cotton has performed better than other varieties. Having two years of data for the same farmers allows innate efficiency differences, due to factors such as farm size, to be separated from the effects of the new technology, which is not normally possible. Farmers who adopted Bt cotton in 1999-2000 benefited according to all the measures used. Higher yields and lower chemical costs outweighed higher seed costs, giving higher gross margins. These measures showed negative benefits in 1998-99, which conflicts with continued adoption, but stochastic efficiency frontier estimation, which takes account of the labor saved, showed that adopters averaged 88% efficiency, as compared with 66% for the non-adopters. In 1999/2000, when late rains lowered yields, the gap widened to 74% for adopters and 48% for non-adopters.