Transmission electron microscopy has been used to determine the interfacial structure of the broad faces of γ (h.c.p.) plates precipitated from the α (f.c.c.) matrix of an Al-15 wt. % Ag alloy. This structure was found to be of the dislocation type. Partial dislocations, of principally edge orientation, with Burgers vectors of the form a/6〈112〉, are very much the dominant component of the interfacial structure. Particularly after relatively long aging times, these dislocations are arrayed in three principal types of configuration: (1) parallel dislocations all having the same Burgers vector; (2) a grid formed by dislocations with two different Burgers vectors; and (3) hexagonal arrays in which three different Burgers vectors are represented. Application of a dislocation strain energy analysis due to Winchell indicates that the hexagonal dislocation array has a slightly lower interfacial free energy than the more frequently found two-dislocation structure. All of the Burgers vectors in these structures are parallel to the broad faces of the γ plates, and thus all of the dislocations are sessile with respect to the migration of the interphase boundary. Moreover, since the partiais can neither climb out of the boundary when in edge orientation nor glide out when oriented as screws, the boundary structure must be wholly immobile. Some other mechanism, probably the formation and lateral movement of ledges, must therefore be responsible for the thickening of y plates. These results confirm the predictions of a general theory of precipitate morphology [H. I. Aaronson, Decomposition of Austenite by Diffusional Processes, p. 387. Wiley (1962)]. Although the dislocation structure of the broad faces of γ plates is sessile with respect to transformation a martensitic surface relief was found to accompany the formation of γ plates at a free surface.