Hi Sean, > But MASON already has a procedure for doing exactly that: a ParallelSequence. ahh, somehow I didn't spot that, but that seems very much to be what I'm looking for ! Just one thing, I had a quick look in the manual and where the ParallelSequence constructor method is described is says "the array ought not be modified later". But my agents can get killed during the simulation, so I have to update ParallelSequence correspondingly. Since ParallelSequence is a subclass of Sequence I thought I could use those methods to change the content of my ParallelSequences !?? Many thanks, Axel