The research reported in this paper explores which variables predict native and non-native speaking teachers' perception of fluency and distinguish fluent from non-fluent L2 learners. In addition to traditional measures of the quality of students' output such as accuracy and lexical diversity, we investigated speech samples collected from 16 Hungarian L2 learners at two distinct levels of proficiency with the help of computer technology. The two groups of students were compared and their temporal and linguistic measures were correlated with the fluency scores they received from three experienced native and three non-native speaker teacher judges. The teachers' written comments concerning the students' performance were also taken into consideration. For all the native and non-native teachers, speech rate, the mean length of utterance, phonation time ratio and the number of stressed words produced per minute were the best predictors of fluency scores. However, the raters differed as regards how much importance they attributed to accuracy, lexical diversity and the mean length of pauses. The number of filled and unfilled pauses and other disfluency phenomena were not found to influence perceptions of fluency. (Contains 4 tables.)