Limites de représentations cristallines

15 pages, in FrenchInternational audienceWe provide an answer to two questions of Fontaine (in the unramified case). First, we show that a limit of crystalline representations, of bounded Hodge-Tate weights, is itself crystalline. Second, we show that every admissible filtered $\phi$-module "comes from" a $(\phi,\Gamma)$-module of finite q-height