A locking mechanism (50) for an air handling duct (10) which includes a plurality of interconnected modules which together made up the duct. The locking mechanism includes a male retainer contained on a frame member of a first module, with the male retainer having a locking arm (54) adapted to be moved from an unlocked position to a locked position. A complementary female retainer (46) is contained on a frame member of a second module, with the female retainer being in the form of an elongated keyway designed to receive and hold the locking arm in a locked position. Means (62) are provided to move the arm from an unlocked position to a locked position when the male and female retainers are in alignment for locking the modules together.