Delta Air Lines added 10 Boeing 737-900ERs to its orderbook during the first quarter, bringing its total orders for the type to 130 aircraft.
The Atlanta-based carrier disclosed the order for incremental aircraft in a quarterly financial filing 12 April.
The order is likely new as Delta had no options for the 737 prior to disclosure, the Flight Fleets Analyzer shows.
Delta has ordered 737-900ERs twice before: 100 in August 2011 and an additional 20 in December 2015.
The 737s have replaced ageing Boeing 757-200s in the airline's fleet.
Delta did have "substitution rights" to replace its order for 18 Boeing 787-8s, which it cancelled in December 2016, with the 737-900ER. However, it did not record any change to its orderbook for the narrowbody at the end of last year.
The carrier was not immediately available to comment on the incremental order.