Purpose: The purpose of this study was to examine the test-retest and inter-rater reliability of a measure of muscle extensibility developed by Tardieu, de la Tour, Bret, and Tardieu (1982) in fullterm and preterm newborns. Method: Twenty-one fullterm infants and twenty preterm infants were examined by two physical therapists. Each physical therapist measured A[subscript O] (shortened position of the muscle belly and lengthened tendon) and A[subscript Max] (maximum muscle belly and tendon length) of the gastrocnemius/soleus muscle twice in succession. Reliability was assessed using intraclass correlation coefficients (2,2) and (3,2). Results: Inter-rater reliability between the two examiners ranged from 0.86 to 0.97 and test-retest reliability on the two measures ranged from 0.91 to 0.98. Conclusions: The results suggest that this measure of muscle extensibility is reliable in the gastrocnemius/soleus muscle with fullterm and preterm newborns. Further research is needed to investigate if differences in muscle extensibility are present between fullterm and preterm infants and the relationship between muscle extensibility and active movement. (Contains 2 figures and 3 tables.)