It has been suggested that energetic ions observed in the magnetosheath may be due to direct leakage of trapped magnetospheric ions. To test this hypothesis, we utilize three-dimensional ion spectra from the energetic particle experiment on Isee 1 for a magnetopause crossing on November 10, 1977. Three-dimensional distribution functions in the magnetosphere and in the sheath are constructed from the observed data. Using the observed magnetic field, a simple one-dimensional model of the magnetopause is developed. Ions are then followed in the model, starting in the magnetosphere, through the magnetopause and ending up in the sheath. Using Liouville's Theorem a model sheath distribution function is then built up by following the magnetospheric distribution function through the model fields. The model distribution is then compared with the observed sheath distribution. For this case we find that the main features of the observed ions in the sheath are consistent with direct leakage and with no energization or de-energization processes, and an inward pointing normal component is required. The model results are sensitive to the direction of the normal component, but not to its magnitude. The energetic particles we map in this case apparently follow a flux tube which does not penetrate the magnetopause where local tangential electric fields have been reported.