Malila is a Bantu language, classified by Guthrie as M.24, spoken in the Mbeya region in S.W. Tanzania. This study focuses on vowel-length alternation in the augment or augment-plus-prefix combinations of nouns in their citation forms. The clearest examples of this vowel-length alternation are found in classes 5 and 9/10, where 70% begins with a short vowel and 30% with a long vowel. The question is which of the two possibilities should be considered as the basic form. The issue is looked at from both angles, and it appears that the forms with the long vowels are considered as basic, with a constraint against long vowels in certain specific environments. This hypothesis is corroborated by examples of vowel shortening in other contexts. This way, most nouns in their citation form have a minimum of four morae, which is necessary to differentiate the four contrastive tonal melodies on nouns. Finally, these constraints and the four-morae minimality requirement may have led to augment-vowel lengthening in a number of nouns with monomoraic roots in other noun classes. Monomoraic nouns in classes 5 and 9/10 always have long augment-prefix vowels, yielding trimoraic surface forms, which cannot be further lengthened, and which have subsequently only two contrastive tonal melodies.