  We propose a hypothesis of the truncation of stellar discs based on the
magnetic model of the rotation curve of spiral galaxies. Once the disc had
formed and acquired its present structure, approximately, three balanced forces
were acting on the initial gas: gravity and magnetic forces, inwards, and
centrifugal force. When stars are formed from this gas, the magnetic force is
suddenly suppressed. Gravitation alone cannot retain the newly-formed stars and
at birth places beyond a certain galactocentric radius they escape to
intergalactic space. This radius is the so-called ``truncation radius'', which
is predicted to be at about 4-5 disc radial scale lengths, in promising
agreement with observations.
