Mirror mode (MM) structures are identified in the Voyager 1 heliosheath magnetic field data. Their characteristics are: 1) quasiperiodic structures with a typical scale size of ∼57 &rgr;p (proton gyroradii), 2) little or no angular changes across the structures (∼3° longitude and ∼3° latitude), and 3) a lack of sharp boundaries at the magnetic dip edges. It is proposed that the pickup of interstellar neutrals in the upstream region of the termination shock (TS) is the likely cause of MM instability during intervals when the IMF is nearly orthogonal to the solar wind flow direction. Concomitant (quasiperpendicular) shock compression of the MM structures at the TS and additional injection of pickup ions (PUIs) throughout the heliosheath will enhance MM growth.