Delta shares ended down 2.61 percent at $45.11 on the New York Stock Exchange.
Stock prices across the industry also dropped, including those of United Continental Holdings Inc, American Airlines Group Inc and Southwest Airlines Co.
Delta blamed slower-than-expected improvement in average fare prices for flights booked at the last minute for the decline in its unit revenue outlook.
Other U.S. airlines are struggling to increase unit revenue as low-price carriers such as Spirit expand domestic flights and upstarts such as Norwegian Air cut trans-Atlantic fares.
Southwest Airlines last month lowered its expectation for first-quarter operating unit revenue after "unexpected softness" in demand for last-minute flight bookings in the second half of February.
In March, the world's largest airline, American Airlines, cut its unit revenue forecast for the first quarter, mainly because it canceled fewer flights than the previous year.