The Bx18A and Bx18B loop route begins at 170th Street and Grand Concourse near the 170th Street station . Then it continues on through the street line until at the 170th Street-Jerome Avenue station. Turning around against to loops. The Bx18A goes through Macombs Road then turns slightly left to Undercliff Avenue. The Bx18B does the same thing at Macombs but it only turns slightly left to Sedgwick Avenue. Those two bus routes finally meet again via 168th Street. And lastly, their destination is at 170th Street and Grand Concourse near the 170th Street station as of an clockwise.

The Bx18 started as a single route on June 22, 1941. The original eastern terminus was at Claremont Parkway-Webster Avenue. Service was cut back to its current terminus in July 1974. Original western terminus was at Andrews Avenue-West 175th Street. Extended west via the former Bx39 service (which was discontinued in 1953) to the River Park Towers in July 1974. On June 26, 2022, the route was extended south from River Park Towers to 168th Street and east on 168th Street, Shakespeare Avenue, and 170th Street to the Grand Concourse, making it as of today a loop route. On January 8, 2023, the Bx18 was split into the Bx18A and Bx18B to label each loop.^[2][3][4] From September 24, 2023, for either 6 or 12 months, a pilot fare-free program is in effect for both loops (one route was chosen from each borough).^[5]