Abstract In this paper, we investigate segmental prolongation in a corpus of spontaneous Japanese monologues consisting of over 700,000 words. We examine effects on the rate of prolongation of various factors including speech types, the genders of speakers, word classes, word positions in the phrase and in the inter-pausal unit, and the presence of preceding fillers. Based on the empirical findings, we state some sirategies in prolonging speech segments used by Japanese speakers.