Molecular dynamics simulations of the diffusion coefficient of systems of polydisperse chains are presented. Each system consists of two lengths of chain of chemically identical flexible polymers. The mean square displacement of the center of mass of each species is measured as a function its length and volume fraction in the blend. The polymer lengths range from N = 10 monomers per chain to N = 90, about three times the entanglement length. The polymer species that comprises the bulk of the melt shows little change in behavior regardless of the length of polymer which makes up the remainder. By contrast, when a species is the minority component, its motion is significantly affected by the length of the matrix chains. When a chain is immersed in a matrix of longer chains, its diffusion coefficient is smaller than its monodisperse value; conversely when a chain is in a blend of shorter chain its diffusion coefficient increases compared to a monodisperse melt. For chains shorter than the entanglement length, the diffusion coefficient compares well to theoretical predictions. The scaling exponent of the mean square displacement of the longest polymer is found to be sublinear, unless blended with very short polymers. The scaling exponent seems to be a measurement of the entanglements that the long polymers experience. (C) 2000 American Institute of Physics. [S0021-9606(00)52106-0].