  Massive disk fragmentation has been suggested to be one of the mechanisms
leading to the formation of giant planets. While it has been heavily studied in
quiescent hydrodynamic disks, the effect of MHD turbulence arising from the
magnetorotational instability (MRI) has never been investigated. This paper
fills this gap and presents 3D numerical simulations of the evolution of
locally isothermal, massive and magnetized disks. In the absence of magnetic
fields, a laminar disk fragments and clumps are formed due to the effect of
self--gravity. Although they disapear in less than a dynamical timescale in the
simulations because of the limited numerical resolution, various diagnostics
suggest that they should survive and form giant planets in real disks. When the
disk is magnetized, it becomes turbulent at the same time as gravitational
instabilities develop. At intermediate resolution, no fragmentation is observed
in these turbulent models, while a large number of fragments appear in the
equivalent hydrodynamical runs. This is because MHD turbulence reduces the
strength of the gravitational instability. As the resolution is increased, the
most unstable wavelengths of the MRI are better resolved and small scale
angular momentum transport starts to play a role: fragments are found to form
in massive and turbulent disks in that case. All of these results indicate that
there is a complicated interaction between gravitational instabilities and MHD
turbulence that influences disk fragmentation processes.
