Recent research in second language acquisition suggests that a number of speed, breakdown, repair, and composite measures reliably assess fluency and predict proficiency. However, there is little research evidence to indicate which measures best characterize fluency at each assessed level of proficiency and which can consistently distinguish one level from the next. This study investigated fluency in 32 speakers' performing 4 tasks of the British Council's Aptis Speaking Test, which were awarded 4 different levels of proficiency (Common European Framework of Reference for Language levels A2-C1). Using PRAAT, the performances were analyzed for various aspects of utterance fluency across different levels of proficiency. The results suggest that speed and composite measures consistently distinguish fluency from the lowest to upper-intermediate levels (A2-B2), and many breakdown measures differentiate between the lowest level (A2) and the rest of the proficiency groups, with a few differentiating between lower (A2, B1) and higher levels (B2, C1). The varied use of repair measures at different levels suggests that a more complex process is at play. The findings imply that a detailed micro-analysis of fluency offers a more reliable understanding of the construct and its relationship with assessment of proficiency.