According to Systemic Functional Linguistic (SFL) theory the structural shape of the clause in English is determined by the three metafunctions — ideational, interpersonal and textual (Halliday 1994:179). In Japanese, the situation is similar as far as ideational (Teruya 1998) and interpersonal (Fukui 1998) meanings are concerned. With respect to the textual metafunction, however, the situation appears to be different. Due to the presence of ellipsis, both anaphoric Subject ellipsis and formal exophoric Subject ellipsis (Hasan 1996), along with the operation of clause chaining, Japanese appears to organise textually over another kind of unit, the Theme unit. This paper will explore the Theme unit as it functions to organise discourse in Japanese, offering grammatical and semantic recognition criteria within a Systemic Functional theoretical framework. Justification for the theorisation of this textual unit will be presented together with a number of examples. In Japanese, the Theme unit is the unit within which Theme and Rheme unfold. Theme is realised by first position in the Theme unit, and the Theme unit can map onto clause simplexes, complexes, clauses within a complex and across sentences (in written texts). The paper will conclude with a discussion of the function of the Theme unit and the nature or status of the Theme unit within the SFL model of language, arguing that the notion is possibly applicable to the analyses of other languages, including English.