莆仙方言前圓唇元音的形成與次方言差異是本文論述重點。前圓唇元音是閩東方言與莆仙方言共有的語音特徵之一，但兩地方言在分布上有異同處，同時莆仙內部次方言也有差異。本文主張前圓唇元音在莆仙的成形與推廣，一開始緣於詞彙借入，因唐宋以來文讀音散播，與閩東方言影響，使得[y]韻進入莆仙方言。而後莆仙進一步推演出圓唇共諧概念，所以閩南的[□]韻，在莆仙相應轉為[□]韻。在引入前圓唇元音後，為了保有原有白讀音位與新進文讀音之間的區辨性，莆仙又以暨有之前圓唇音位特徵衍生出[ø]韻，從而形成現有莆仙方言的各類前圓唇元音。同時，對莆仙韻母的內部次方言差異，可歸納出三類肇因：1) 同一規則在各地演變速率不一；2) 同一規則應用範圍擴大；及3) 為了維持語言內部系統性區辨，致使連續性地調整變化產生。

This paper explains the development of front rounded vowels and its sub-dialectal variations in the Puxian dialect of Min. Front rounded vowels are found in both Eastern Min and Puxian, but their distributions in the two dialects are different; in addition, Puxian also has sub-dialectal differences. In this paper, I argue that the front rounded vowel [y] was introduced to Puxian via lexical borrowing from Northern Guanhua and Eastern Min. Puxian further developed rounding harmony, which resulted in [□] (corresponding to [□] in Southern Min). The feature combination [-back, +round] was also employed to preserve the phonemic contrast of both colloquial and literal strata in the phonological system, and [ø] final was formed then. In other words, front rounded vowels entered the Puxian phonological system at different times and through various routes. I give three factors which influenced the sub-dialectal variations of finals in Puxian: 1) different speed and spread of sound change among sub-dialects, 2) phonological rules which applied to different domains, and 3) maintenance of systemic contrast via successive adaptations of finals in some sub-dialects.