Vygotsky proposed the concept of the zone of proximal development (ZPD) but did not elaborate the concept of "peer," leaving open the question of how capable a peer should be and how practice or outcome might differ according to a peer's level of capability. These issues were investigated in a sociocultural study of first-year Vietnamese university students learning English. The students were examined in two group settings, "unassisted" and "assisted," the first consisting of five students from the same class and the second of four from the same class and a more knowledgeable student from a higher class. The senior student made a marked difference to the organization of the assisted group's processes: she helped the other students start their discussions, explained new English words, and increased the use of English in this group setting. The results suggest that the teaching manner of the senior student contributed substantially to the group work in the current study.