  We present a new very fast tree-code which runs on massively parallel
Graphical Processing Units (GPU) with NVIDIA CUDA architecture. The
tree-construction and calculation of multipole moments is carried out on the
host CPU, while the force calculation which consists of tree walks and
evaluation of interaction list is carried out on the GPU. In this way we
achieve a sustained performance of about 100GFLOP/s and data transfer rates of
about 50GB/s. It takes about a second to compute forces on a million particles
with an opening angle of $\theta \approx 0.5$. The code has a convenient user
interface and is freely available for use\footnote{{\tt
http://castle.strw.leidenuniv.nl/software/octgrav.html}}.
