In many Oceanic languages, subject arguments are indexed by preverbal markers within the verb complex. In Marovo, an Oceanic language of the Solomon Islands, such subject markers show unusual synchronic behavior, in that their presence is conditioned by both morphosyntactic and pragmatic characteristics of the clause. Thus, in Marovo preverbal subject markers occur obligatorily with certain clause-initial discourse connective particles and with the negative particle. Subject markers also occur outside of these morphosyntactic environments, where their use contrasts with that of other expressions of the subject argument, including lexical or pronominal noun phrases or the lack of overt expression. Within this context of subject expression more generally, the occurrence of Marovo subject markers can be seen to be determined by the discourse role of the subject argument. It is argued here, through comparison of Marovo subject marking with that in closely related languages, that these synchronic conditions have diachronic explanations.