The present study investigated whether Japanese infants are sensitive to prosodic cues to clausal units in native (Japanese) and non-native (English) languages. Two types of speech samples (child-directed speech) of each language were constructed for use as stimulus materials: Coincident samples were created by inserting 1-s pauses at all clause boundary locations, and noncoincident samples were created by inserting the same number of pauses between words within clauses. Groups of 6- and 10-month-old infants were tested using Japanese samples, and groups of 4- and 10-month-old infants were tested using English samples on the headturn preference procedure. The results showed that only the 10-month-old infants tested on Japanese speech samples demonstrated a significant preference for the noncoincident samples over the coincident samples. These results indicate that very young Japanese infants are not sensitive to clausal units in either language, but they come to be sensitive to clausal units only in their native language before they are 10 months old.