This dissertation is a study of the rules of customary behaviour (āsamudācārika-dharmas) found in a North Indian Buddhist monastic law code, the Mūlasarvāstivāda-vinaya. Other than Gregory Schopen, few scholars have noted the significance of these rules. Schopen points out that according to this vinaya, adherence to rules of customary behaviour is foundational for achieving nirvāṇa. Yet, these rules have been practically ignored in contemporary scholarship. Building on Schopen’s work, I approach this material with two main questions: 1) What are rules of customary behaviour? and 2) How do rules of customary behaviour function in the Mūlasarvāstivāda-vinaya? In an attempt to answer these questions, I explore passages from the Mūlasarvāstivāda-vinaya in which the Buddha prescribes rules of customary behaviour for specific monastics in response to a variety of narrative situations. I organize this material into three thematic chapters. First, I discuss rules of customary behaviour related to the administration of the monastic community (saṃgha). Next, I explore rules of customary behaviour relevant only in specific environments. Finally, I investigate rules of customary behaviour prescribed in response to illnesses in the saṃgha. Through the above exploration, I demonstrate three main points: 1) that there are three ways that rules of customary behaviour appear in the Mūlasarvāstivāda-vinaya; 2) the adoption of rules of customary behaviour prescribed in narratives in the Mūlasarvāstivāda-vinaya does not necessarily signal the creation of a new monastic office or official duty; and 3) In the vast majority of cases, these rules seem to be designed to protect the integrity of the saṃgha and accommodate monks or nuns who are experiencing temporary challenges to their ecclesiastical status. / Dissertation / Doctor of Philosophy (PhD)