South Efate, an Oceanic language of central Vanuatu, allows the expression of temporal relations in several ways using markers of aspect and mood. Pronominal expression of arguments is obligatory and, as subject proclitics occur in one of three forms (realis, irrealis, and perfect), expression of aspect or mood is required in every sentence. South Efate is one of a group of Vanuatu languages that displays stem-initial mutation, whereby the initial consonant of a very small group of verbs changes to reflect mood. This paper presents evidence that fortis (realis) and lenis (irrealis) stem mutation also correlates with features of transitivity, not a surprising finding following the work of Hopper and Thompson. All else being equal, the fortis form of the verb occurs in clauses that have an overt expression of an object, while the lenis form occurs when there is no object in the clause. A further curiosity is that stem-initial mutation has been maintained for just a small class of verbs, so its correlation with transitivity in just this small class is all the more interesting. This paper explores the relationship between the morphological expression of mood and transitivity in South Efate, and suggests frequency of use as an explanation for the retention of this marginal system that affects only 7 percent of verb stems in the lexicon.