Crystalline solid forms of the selective late sodium current inhibitor 4-(pyrimidin-2-ylmethyl)-7- (4-(trifluoromethoxy)phenyl)-3,4-dihydrobenzo[f][l,4]oxazepin-5(2H)-one (Compound I) were prepared and characterized in the solid state: Also provided are processes of manufacture and methods of using the crystalline forms.