We present the results of high resolution SPH simulations of
gravitationally unstable protoplanetary gaseous disks. Disks as massive as
0.1 solar masses out to 20 AU develop strong spiral structure in the
outer, colder regions.
If cooling is efficient and the temperature of the disk remains below 60 K
in these regions, overdensities form along the spiral arms
that collapse into proto-Jupiters in a few hundred years.
We show that this occurs even when cooling is switched off within the
overdense regions. We use lower resolution simulations to follow the
dynamical evolution of the protoplanets for up to
a 1000 years; systems with 2 or 3 giant planets result in the end
with  masses and orbital eccentricities resembling those of observed
extrasolar planets. Hence the gravitational instability seems a viable
mechanism to form giant planets.