Robert Blust in this journal in 1996 drew attention to a process in a number of widely separated Oceanic languages in which the first a of an aCa sequence dissimilates to a higher vowel. In some of these languages the rule is still productive, while others show only historical dissimilation. He briefly presented data from three areas in Vanuatu comprising ten languages. More recent research, however, shows that there are about thirty Vanuatu languages that exhibit this phenomenon, though only four seem to do so as part of their synchronic morphophonemics. While an initial analysis suggests that there are four different types of dissimilatory processes, three of these (accounting for all but three or four of the thirty languages) can be shown to be derivable from a single historical process in which dissimilation applied iteratively from left to right: *a, whether stressed or unstressed, developed the allophone [e] as a result of this, though dissimilation was apparently blocked by intervening or adjacent postvelars, velars, and labiovelars. In languages that reflect the dissimilation, this [e] is now phonemic /e/, /e/, or less frequently /i/. (The fourth type is dependent on stress, but is almost certainly a more recent development in just one small geographical area.) I propose that dissimilation occurred in the ancestor of all languages from Malakula and Ambrym in the north to Aneityum in the south, and that the nondissimilating languages within this region have subsequently reversed the process, allophonic [e] becoming [a]. A brief comparison is made with similar cases in Micronesia, but it seems impossible, on the basis of data available, to suggest any historical connection between the two regions.