This paper examines the impact of transgenic technology adoption on varietal diversity. Transgenic pest-resistant varieties are hypothesized to reduce farmers’ demand for on-farm diversity through an act of substitution, as both serve as production risk reducing instruments. This adverse agro-biodiversity impact might be partially counteracted by an expanding seed sector, supplying a large number of transgenic varieties. The case of Bt cotton in India is taken for empirical illustration. The production function analyses show that both Bt technology and on-farm varietal diversity enhance yield, while reducing the production risk. With few Bt varieties available in the first years, technology adoption entailed a reduction in on-farm varietal diversity. This effect, however, was partially offset by more Bt varieties becoming available over time.