Vygotskian sociocultural theory of mind holds that language mediates thought. According to the theory, speech does not merely put completed thought into words; rather, it is a tool to refine thought as it evolves in real time. This study investigated from a sociocultural theory of mind perspective how nine beginning learners of Latin used private speech to supplement different types of computer-based learning support in their efforts to self-regulate, or internalize and apply new linguistic knowledge, at the initial stages of learning how to distinguish thematic agent/patient roles in Latin. Analysis of features of participants' private speech in combination with their learning outcomes showed that successful self-regulation is possible in the context of independent, computer-based learning, and with minimal support from the learning environment, but that for some learners to gain control of the learning task, more responsive, adaptable learning support appeared to be a "sine qua non" condition of successful self-regulation.