  We study the intermittency and field-line structure of the MHD turbulence in
plasmas with very large magnetic Prandtl numbers. In this regime, which is
realized in the interstellar medium, some accretion disks, protogalaxies,
galaxy-cluster gas, early Universe, etc., magnetic fluctuations can be excited
at scales below the viscous cutoff. The salient feature of the resulting
small-scale magnetic turbulence is the folded structure of the fields. It is
characterized by very rapid transverse spatial oscillation of the field
direction, while the field lines remain largely unbent up to the scale of the
flow. Quantitatively, the fluctuation level and the field-line geometry can be
studied in terms of the statistics of the field strength and of the field-line
curvature. In the kinematic limit, the distribution of the field strength is an
expanding lognormal, while that of the field-line curvature K is stationary and
has a power tail K^{-13/7}. The field strength and curvature are
anticorrelated, i.e. the growing fields are mostly flat, while the sharply
curved fields remain relatively weak. The field, therefore, settles into a
reduced-tension state. Numerical simulations demonstrate three essential
features of the nonlinear regime. First, the total magnetic energy is equal to
the total kinetic energy. Second, the intermittency is partially suppressed
compared to the kinematic case, as the fields become more volume-filling and
their distribution develops an exponential tail. Third, the folding structure
of the field is unchanged from the kinematic case: the anticorrelation between
the field strength and the curvature persists and the distribution of the
latter retains the same power tail. We propose a model of back reaction based
on the folding picture that reproduces all of the above numerical results.
