Harp traps are specially designed tools to capture bats exiting roost structures in numbers that make mist nets impractical or unsafe for the bats. Harp traps are comprised of a frame with monofilament (fishing line) strung vertically at a spacing of about 1 inch between strands. Two frames are arranged side-by-side and placed in a flyway (often as bats exit a roost such as a cave). Bats collide with the monofilament and tumble, unharmed, into a catch bag at the bottom of the trap that prevents the bats from escaping. Removal of bats from the catch bag is quick and minimizes stress on the captured bats.

Some anecdotal evidence suggests that certain species, such as the Eastern small-footed bat (Myotis leibii) may be more likely to be captured in harp traps compared to the more traditionally-used mist nets.

Harp traps are excellent tools to use for cave surveys or mine portal surveys due to their ability to deal effectively with large numbers of bats emerging in a short time period. They can also work well with bats emerging from hollow roost trees, such as trees used by Rafinesque's big-eared bats (Corynorhinus rafinesquii) and southeastern myotis (Myotis austroriparius). The trap above is being used in a bottomland hardwood forest to capture southeastern myotis bats. The plastic sheeting is acting to funnel the bats from the roost opening (near ground level) into the trap.

The video at the top of the page illustrates the speed at which bats can emerge from roost structures, and in many cases they may emerge much more quickly!