Bacillus thuringiensis (Bt) maize is the first genetically modified crop to be introduced to Philippine farmers. Since its commercialization in 2003, evidence has accumulated regarding the economic benefits of adopting Bt maize. Based on data collected from 466 farmers, this research focuses specifically on the selection and endogeneity bias that has not yet been explicitly addressed; it also tests the impacts of adoption on net farm income, off-farm income of the farmer, and total household income, including income from crops and livestock, off-farm income of the farmer, and income of all household members. Regression results are compared for OLS regress and two types of instrumental variables methods. Accounting for self-selection and endogeneity, adoption of Bt maize increases net farm income, off-farm income, and household income. The predicted probability that a household’s income falls below the poverty line is lower among Bt maize adopters. However, we have not fully addressed placement bias.