This study investigated whether lower vagal function in preterm infants is associated with increased fundamental frequency (F-0; frequency of vocal fold vibration) of their spontaneous cries. We assessed respiratory sinus arrhythmia (RSA) during quiet sleep as a measure of vagal function, and its relationship with the F-0 of spontaneous cries in healthy preterm and term infants at term-equivalent age. The results showed that preterm infants have significantly lower RSA, and higher overall F-0 than term infants. Moreover, lower RSA was associated with higher overall F-0 in preterm infants, whereas higher RSA was positively associated with mean and maximum F-0, and a larger F-0 range in term infants. These results suggest that individual differences in vagal function may be associated with the F-0 of spontaneous cries via modulation of vocal fold tension in infants at an early developmental stage. (c) 2016 The Authors. Developmental Psychobiology Published by Wiley Periodicals, Inc. Dev Psychobiol 58:724-733, 2016.