Abstract: A rubber-modified, rigid, heat- and impact resistant polymer composition comprising:(a) an interpolymer comprising monovinylidene aromatic monomer, maleimide monomer and ethylenically unsaturated nitrile monomer polymerized therein; and(b) from about 7 to about 25 weight percent of a first graft copolymer prepared by an emulsion process and comprising from about 30 to about 70 weight percent of a copolymer (A) comprising in polymerized form from about 10 to about 60 weight percent of an ethylenically unsaturated nitrile monomer and from about 40 to about 90 weight percent of a copolymerizable monovinylidene aromatic monomer, the copolymer (A) being grafted at least in part to a rubbery substrate polymer (B) which is from about 30 to about 70 percent of the graft copolymer by weight and has a rubber particle size distribution wherein the average particle size is from about 0.05 to about 0.