This article proposes that the present tense forms of the be-auxiliary byti (AUX) in Old Russian should be identified as weak pronouns, based on their prosodic, morphological, and syntactic properties. The loss of AUX in Old Russian, which distinguishes East Slavic from West and South Slavic languages, is ascribed to the competition between AUX and strong subject pronouns as a consequence of the reanalysis of AUX as a weak subject pronoun. This reanalysis was triggered by the early loss of synthetic past tenses in Old Russian. The AUX's position to the right of pronominal clitics indicates that the AUX remains in its initial merge position either as an auxiliary verb or as a subject pronoun. The co-occurrence of AUX in situ and a null subject in Old Russian implies that rich verbal agreement and null subjects are unrelated with verb raising to I 0. *I thank Henning Andersen, Wayles Browne, Steven Franks, Ora Matushansky, Krzysztof Migdalski, and many others, who gave me helpful comments while I developed this project. Some parts of this work have been presented at conferences including Formal Approaches to Slavic Linguistics 23 (UC Berkeley, 2014). 1 Zaliznjak (2008:248, 260-262) observes that the auxiliary system began to visibly change during the second half of the 16 th century and that the auxiliary was practically out of use in spoken language in the 17 th century.