This research attempts to clarify the characteristics of the “poor” households in Phu An Commune, located on the shore of Tam Giang Lagoon, Central Vietnam. To understand the socioeconomic background of poverty in the area, the authors examine the changes in agriculture as well as shrimp aquaculture since the adoption of “Doi Moi”, an open-door policy, in 1986. Then, the constraints which make it difficult for them to escape from poverty are discussed. The results of this study show that agricultural and aquacultural changes have diversified the locals' livehoods. The positive changes are reflected in the decline of the number of “poor” households as well as in their evaluation of their lives compared with 20 years ago. Nevertheless, there are still many constraints which cause the “poor” households to remain poor in spite of the remarkable economic growth during the last 20 years. Insight into the detailed situation of poverty in the area is given through the categorization of the residents by their livelihood: mobile gear fishing (usually called “Sampan people”), fixed gear fishing, farming, farming and fishing, and service. In the process of the “Doi Moi” policy and following the introduction of shrimp aquaculture around 1999, the water surface of Tam Giang Lagoon, which used to be an open-access common resource, was allocated to individuals for their exclusive use and exploited mainly for shrimp aquaculture. Due to these drastic changes in local resource, the “Sampan people”, who have been engaged in only fishery in Tam Giang Lagoon, have changed their position in the commune from the greatest beneficiaries to those of sacrifices. Now the poorest in this area are those who engage in mobile gear fishing, the “Sampan people”.