This paper uncovers a systematic correlation between semantics of aspect and syntactic argument structure as manifested in the difference between two imperfective aspect markers -ko iss and -a iss in Korean. Unlike the common assumption that the -ko iss form is a progressive marker, while the -a iss form is a resultative marker, this paper argues that the difference between the two derives from their different argument structure: -ko iss selects transitive and unergative verbs, which have an external argument, while -a iss selects unaccusative and passive verbs, which only have an internal theme argument. It is argued that the difference in argument structure is determined by semantic event structure depending on agentivity in Korean. The results of the paper have broader implications for the issues of syntax and semantics interface and unaccusativity.