Abstract: Rubber-modified, impact-resistant polystyrene resin compositions having a well balanced combination of improved properties such as tensile strength and tensile elongation as well as reactivity with styrene monomers, impact resistance, low-temperature properties, and gloss, comprise dispersed particles of a rubber polymer which satisfies the following: (1) the content of the rubber polymer in the compositions is from 1 to 25% by weight; (2) the rubber polymer is a high-cis high-vinyl polybutadiene comprising from 65 to 95% cis-1,4-structures and from 30 to 4% 1,2-structures; and (3) the relationship between the 1,4-structures and 1,2-structures of the rubber polymer is represented by the following expression (A), wherein the value of .beta. is in the range of 1.0<.beta..ltoreq.1.43;.beta.=P.sub.1,2-,1-4 /(2.times.P.sub.1,2 .times.P.sub.1,4)(A)wherein P.sub.1,2 represents the content of 1,2-structures, P.sub.1,4 represents the content of 1,4-structures, and P.sub.