... We present a new code for evolving three-dimensional self-gravitating collisionless systems with a large number of particles . FLY (Fast Level-based N-bodY code) is a fully parallel code based on a tree algorithm. ... FLY output data can be analysed with AstroMD, an analysis and visualization tool specifically designed for astrophysical data. ... During the tree walk procedure, FLY builds a single interaction list (IL) to be applied to all particles inside a grouping cell ( ). ...