This study, framed within sociocultural theory, examines the interaction of adult ESL test-takers in two tests of oral proficiency: one in which they interacted with an examiner (the individual format) and one in which they interacted with another student (the paired format). The data for the eight pairs in this study were drawn from a larger study comparing the two test formats in the context of high-stakes exit testing from an Academic Preparation Program at a large Canadian university. All of the test-takers participated in both test formats involving a discussion with comparable speaking prompts. The findings from the quantitative analyses show that overall the test-takers performed better in the paired format in that their scores were on average higher than when they interacted with an examiner. Qualitative analysis of the test-takers' speaking indicates that the differences in performance in the two test formats were more marked than the scores suggest. When test-takers interacted with other students in the paired test, the interaction was much more complex and revealed the co-construction of a more linguistically demanding performance than did the interaction between examiners and students. The paired testing format resulted in more interaction, negotiation of meaning, consideration of the interlocutor and more complex output. Among the implications for test theory and practice is the need to account for the joint construction of performance in a speaking test in both construct definitions and rating scales. (Contains 2 figures, 4 tables, and 5 footnotes.)