Resistance against insect pests is one of the most promising and highly favored characteristics of an economically important crop. In the Philippines, the Asian corn borer (ACB), Ostrinia furnacalis (Guenée), is the most destructive insect pest of maize. The discovery of Bacillus thuringiensis and the Bt toxin, which led to the development of Bt corn, effectively controlled this pest. In this study, four traditional varieties of maize that showed resistance against ACB were tested for the presence of Bt Cry1Ab protein. Out of these four, a traditional variety from San Carlos City, Pangasinan, which was initially thought to have natural resistance against the pest, tested positive for the presence of the Cry protein. Confirmatory testing for the presence of the transgene was done through PCR, which also confirmed the locations in Pangasinan where introgressed Silangan are present. This is the first report of natural introgression of cry1Ab in a traditional maize variety in the Philippines. Results of this study can be used as basis to profile the existing CGUARD collection of traditional maize for the presence of transgenes, and in future corn germplasm conservation and utilization efforts. These can also be used to suggest improvement on the existing policy on biosafety measures to be implemented in the country.