War in Iraq and severe acute respiratory syndrome have slowed air travel.
United spokesman Chris Brathwaite says the airline is matching Delta's prices in competing markets. Northwest also matched the sale in such markets.
American Airlines matched the fare sale Monday.
Some of the tickets are as low as $44 one-way. They require a week's advance purchase.
The fares are valid for travel Monday through Thursday and on Saturday from April 14 through Oct. 1. The lower fares are unavailable for most holiday periods.
Tickets must be purchased by April 16.