This paper investigates principles which govern the various realisations of the present progressive aspect marker -tu- in Malawian Tonga. It is realised as zero when the morphological stem involved has two or more syllables. It is realised as -tu- when the stem involved is monomoraic. -tu- also surfaces before object markers. It is also realised as -t- or -tw- when it precedes vowel-initial verb stems. The main claim made in the paper is that -tu- is a lexical foot-head and a bona fide marker of the present progressive aspect. Since all foot-heads in Bantu languages are required to belong to the prosodic stem, its deletion when it falls before verb stems with two or more syllables ensures that there is no foot-head falling outside the prosodic stem. Preservation of -tu- before monomoraic verb stems and object markers is attributed to the fact that the foot-head -tu- in these forms is now part and parcel of the disyllabic prosodic stem. Finally, the realisation of -tu- as -t- or -tw- before vowel-initial verb stems ensures that prosodic stems have proper onset.