Final Syllable Lengthening (FSL) has been extensively examined in infant vocalizations in order to determine whether its basis is biological or learned. Findings suggest there may be a U-shaped developmental trajectory for FSL. The present study sought to verify this pattern and to determine whether vocal maturity and deafness influence FSL. Eight normally hearing infants, aged 0;3 to 1;0, and eight deaf infants, aged 0;8 to 4;0, were examined at three levels of prelinguistic vocal development: precanonical, canonical, and postcanonical. FSL was found at all three levels suggesting a biological basis for this phenomenon. Individual variability was, however, considerable. Reduction in the magnitude of FSL across the three sessions provided some support for a downward trend for FSL in infancy. Findings further indicated that auditory deprivation can significantly affect temporal aspects of infant speech production.