• Histometric data on muscle fiber types were studied in two cases of congenital myotonic dystrophy (CMD); one underwent biopsy at the age of 5 months and the other at the age of 10 years. A previously unreported severe deficiency of type IIB fibers were found in both cases. In addition, in the first case, there was type I fiber preponderance and hypotrophy as described in cases of congenital fiber type disproportion (CFTD). It is suggested that an abnormality of motor unit maturation may be common to CMD and to CFTD, and that this results from a disorder of neural trophic influences during muscle development.