This paper aims to reveal the process of formation of fish distribution channels by street traders, with special attention to the transaction network of traders in suburban Hanoi. The transaction record books of middlemen are used for the analysis of relationships between the spatial pattern of the network and aquaculture production.<br> Doi Moi, the economic reform, legalized private business and the sales in marketplaces proliferated. The inspection gates on national roads were removed, which allowed rural people to commute to Hanoi freely. Fish retailers including street traders mainly consist of Hanoi residents and residents of the suburban area 15 to 20km south of the city. Many Hanoi residents were formerly employed in state enterprises. Those who started businesses after the 1990s became street traders, while those started before Doi Moi secured places in fixed marketplaces. Most residents in the suburban area started fish trading in the 1990s. These suburban residents own small rice field plots, but depend mainly on trading for their household income.<br> Street traders living in Hanoi tend to choose marine fish to sell, although freshwater fish are more popular in Hanoi. This tendency is due to complexity in price evaluation and access to sellers in the trade of freshwater fish. As a result, traders rely on one wholesale market for buying fish. On the other hand, suburban traders have a variety of choices of fish and sellers. For these traders, the formation of transaction networks with middlemen and producers is important, which the analysis of trader hierarchy in informal-sector studies have tended to ignore.<br> Aquaculture production of freshwater fish by household units generally employs a polyculture method, that is, the culture of several different species in one pond. Since the culture area is small, the harvest frequency is limited to a few times a year. Each middleman conducts transactions with several producers so that they provide traders with a daily supply. The traders involved in this transaction network are not limited to the middlemen's village or prefecture. The spatial structure of the networks shows that they were developed in response to the various fish species produced in each pond, although they were initially formed on the basis of geographic proximity. For the middlemen to develop such networks, local marketplaces are important for information exchange.<br> It is suggested that the formation process of the transaction relationships of street traders should not be analyzed solely from the viewpoint of a hierarchical structure with a wholesaler at its summit. Street traders have a variety of selling and buying strategies, using commodities and places that are available between their residences and selling places. In the case of Hanoi, where the informal sector consists of many suburban residents, it is important to pay attention to the transaction networks developed through local marketplaces.