Face-centered cubic cobalt films, in which there were thickness gradients, were grown in high vacuum (<5×10-8 torr) on the (001) surfaces of copper films prepared inside the vacuum chamber. Examination of the specimens in an electron microscope-fitted with an X-ray microanalysis attachment-revealed that the cobalt was strained to match the copper lattice until an overgrowth thickness of 18.5 ± 5 Å was reached. This result agrees with theoretical predictions. At 18.5 ± 5 Å, misfit dislocations were formed: some of these were partial dislocations, the remainder were complete. The density of misfit dislocations increased as film thickness increased beyond 18.5 +- Å but did so less rapidly than predicted by theory. It is suggested that this discrepancy between theory and experiment resulted from two processes which impeded the generation of misfit dislocation lines. The first was impaction of partial dislocations against stacking faults on intersecting slip planes and hindered the generation of partial misfit dislocations. The second was interaction between dislocations and hampered the generation of complete misfit dislocation lines. © 1970.