In the context of 90 adult Japanese learners of English with diverse second language experience and 10 native speakers, this study examined the linguistic characteristics and learner profiles of low-, mid- and high-level fluency performance. The participants’ spontaneous speech samples were initially rated by 10 native listeners for global fluency on a 9-point scale (1 = dysfluent, 9 = very fluent), and then divided into four proficiency groups via cluster analyses: low (n = 29), mid (n = 30), high (n = 31), and native (n = 10). Next, the data set was analyzed for the number of pauses within/between clauses, articulation rate, and the frequency of repetitions/self-corrections. According to the results of a series of analyses of variance, the frequency of final-clause pauses differentiated low- and mid-level fluency performance; the number of mid-clause pauses differentiated mid- and high-level performance; and articulation rate differentiated high-level and nativelike performance. The analyses also found that the participants’ second language fluency was significantly associated with their length of residence profiles (0–18 years), but not with their age of arrival profiles (19–40 years).