Dengue virus, a member of the Flaviviridae family, has a surface composed of 180 copies each of the envelope (E) glycoprotein and the membrane (M) protein. The crystal structure of an N-terminal fragment of E has been determined and compared with a previously described structure. The primary difference between these structures is a 10° rotation about a hinge relating the fusion domain DII to domains DI and DIII. These two rigid body components were used for independent fitting of E into the cryo-electron microscopy maps of both immature and mature dengue viruses. The fitted E structures in these two particles showed a difference of 27° between the two components. Comparison of the E structure in its postfusion state with that in the immature and mature virions shows a rotation approximately around the same hinge. Flexibility of E is apparently a functional requirement for assembly and infection of flaviviruses.