The isotropic-nematic (I - N) phase behavior of athermal chain-length polydisperse systems was investigated by molecular simulation. The approach adopted involved the formulation of a novel semigrand Gibbs-type of ensemble for polydisperse chain systems, the use of extrapolation methods (like histogram reweighting) to target polydispersities of interest, and the implementation of both intermutation moves among species and expanded-ensemble moves (for the transfer of the shortest chains) to attain chemical potential equilibration. This approach was first used to illustrate how the results of a system with bimodal polydispersity distribution can be used to get coexistence data for a bidisperse system of short and long chains, thereby circumventing the need to insert/delete the longest molecules. Simulations were then used to examine the effect of osmotic pressure (concentration) and the shape of the chain-length polydispersity function on the I-N phase transitions. In agreement with the predictions of Flory theory, the phase behavior of a unimodal (Poisson) parent distribution showed significant partitioning of the components between the coexisting phases, wherein longer chains concentrate in the nematic phase. Within the biphasic region, the order parameter in the nematic phase increased with pressure despite the growing proportion of shorter chains. Very short chains, like dimers and trimers, were found to be disordered in the nematic phase. Partitioning effects are more pronounced when the parent system had a flatter or a bimodal chain-length distribution. Flory theory gives a good description of the partition effects, but significantly overestimates the coexisting concentrations and the nematic ordering. (C) 2003 American Institute of Physics.