CAIRO, Aug 20 (Reuters) - The average yields on Egypt's three- and nine-month treasury bills fell at an auction on Sunday, central bank data showed.

The average yield on the 91-day bill fell to 19.059 percent from 19.080 at the last similar auction on Aug. 13. The yield on the 273-day bill fell to 18.978 percent from 19.004 percent at the last auction.

(Reporting by Ahmed Aboulenein; editing by Mark Heinrich) ((ahmed.aboulenein@thomsonreuters.com; +20 2 2394 8097; Reuters Messaging: ahmed.aboulenein.thomsonreuters.com@reuters.net))