A simple ballistic model for vacancy migration is presented. An expression for the enthalpy of migration in terms of the interatomic spacing and Debye temperature is derived and evaluated for 62 elemental crystals. Calculated values agree with measured values within 15% for the 11 close-packed crystals where data were found. For Si and Ge they equal high-temperature measurements. The model explains the anomalous suppression of the low-temperature migration process near the melting points of Si and Ge. © 1975 The American Physical Society.