This article explores the “private trade” network of British East India Company merchants in one segment of the eighteenth-century Indian Ocean world, foregrounding a new perspective on this element of European commerce in early modern Asia. Much recent work on this topic emphasizes the importance of the global or “diasporic” connections back to Europe that British merchants made for the success of private trade. By contrast, this article argues for the importance of regionally situated commercial associations for the day-to-day functioning of private trade, particularly emphasizing the role of Asian merchants. While private traders were clearly embedded in epistolary networks that connected them to an emerging global economy, looking at the western Indian Ocean reveals the extent to which they also relied on more localized commercial associations mediated through the exchange of letters. Letter-based ties to local merchants were vital for the exchange of commercial information, for the procurement of goods, and for the transfer of credit. Such synergistic transcultural interactions were central to the emergence, operation, and growth of British private trade networks in the eighteenth century.