A vinylidene chloride copolymer composition is produced by copolymerizing 60 to 95 wt. % of vinylidene chloride and 40 to 5 wt. % of a comonomer in the presence of a rigid polymer which has a softening temperature of 50 to 180.degree. C. and a melt viscosity of 0.1 to 500 poise at 170.degree. C. and is soluble in vinylidene chloride monomer and is not substantially miscible to the vinylidene chloride copolymer, at a ratio of 0.01 to 5 wt. parts per 100 wt. parts of the monomer mixture.