Previous research suggested possible speech-production- and/or speech-perception-oriented causes for the temporal phenomenon referred to as final-syllable vowel lengthening, yet little conclusive evidence was adduced for either possibility. This phenomenon was investigated from a developmental perspective. A much smaller amount of final-syllable vowel lengthening was observed in the premeaningful vocalizations of a number of very young infants than in phonetically comparable utterances produced by adult speakers of English. Extensive final-syllable lengthening observed in the productions of adult speakers of English seemingly constitutes a learned behavior.