This study examines L2 fluency in speaking interactions based on the number and type of utterances. The participants were 25 Korean eighth-grade learners of English as a foreign language. They performed five communicative tasks in groups, and their speaking interactions were audio-recorded and analyzed to measure the frequencies of sentence-level and word-level utterances. Results showed that learners of different levels of L2 fluency greatly varied in their frequencies of sentence-level utterances. Construction-based analyses found that the frequency variation in sentence-level utterances was primarily attributable to the transitive construction and a small set of intransitive constructions. Further investigation of the transitive complementation patterns suggested that L2 learners' use of the nominal complementation [V+NP] became more productive as they expanded their repertoires of transitive complementation in developmental sequence based on a set of complementation clusters. Regarding these acquisitional patterns of the constructions in respect to L2 fluency development, the present study concludes with pedagogical implications and suggestions for future studies.