Delta will not allow pets to travel as checked baggage starting in March, leaving American Airlines as the only major U.S. airline allowing animals to travel in the cargo hold of a passenger airplane.

Beginning March 1, pets that are too large to fit under a passenger’s seat in the main cabin will have to travel as freight through Delta Cargo. Exceptions to the rule include passengers flying Delta One premium service, members of the military with active transfer orders, and service and emotional support animals with proper documentation.

Pets shipped via Delta Cargo are not guaranteed to be shipped on a customer’s same flight or flight schedule and require a separate booking. Delta said the pets are monitored closely during travel and handled in temperature-controlled holding areas at airports and in vans.

American still allows animals to travel as carry-on or checked baggage, or with American Airlines Cargo. Delta’s new guidelines are in line with United’s pet policy; JetBlue and Southwest only allow animals to travel with their owners on the plane.