This paper examines informal Japanese conversations between sixteen pairs of same-sex friends to explore the preferred information structure of the intonation unit (Chafe and Chafe) and the preferred clause structure in terms of the number and type of arguments contained per clause. The results show that in conversational speech (a) Japanese speakers express overtly one NP and introduce one new nominal referent within one intonation unit; and (b) they express overtly one argument, typically S or O, within one clause. The quantity constraints that this study found Japanese speakers conform to are discussed in terms of Du Bois's (1987) Preferred Argument Structure.