I am a U.S. citizen, but also have a foreign passport. Can I travel to the United States on the foreign passport?
No. U.S. citizens must enter and leave the United States on valid U.S. passports, even if they hold a passport from another country. If your U.S. passport has been lost or stolen, or if it has expired, you must apply to replace it before traveling to the United States.
london.usembassy.gov/cons_new/faqs/faq_citizenship.html#003