Cotton, the major cash crop in Pakistan, suffers 30% losses to cotton leaf curl disease, caused by the geminivirus, cotton leaf curl virus DNA A, plus a satellite component, DNA beta responsible for symptom development with plants failing to produce cotton bolls. We constructed transgenic tobacco expressing sense and antisense RNAs representing: [i] the 5' half of the viral DNA replication gene, AC1, [ii] the 3' half of AC1, [iii] two overlapping genes, AC2, a transcription activator, and AC3, a replication enhancer. In contrast to controls, 25% of 72 transgenic tobacco lines tested showed heritable resistance [T(1) - T(3) generations]: symptom-free and no replication of DNA A or DNA beta even after 120 days of continuous exposure to viruliferous whiteflies. As geminiviral and transgene RNAs are not detected in resistant lines following infection, and selected uninfected resistant tobacco sense lines reveal double-stranded and small interfering RNAs, the most likely mechanism is via post-transcriptional gene silencing.