Dynamic messages are first-class messages dynamically bound to program variables.
Dynamic messages are critically needed for some programs, but it seems that there has been no proposal of static type systems for dynamic messages. This paper presents a static typing discipline for dynamic messages and formalizes it into a second order polymorphic type system. The type system satisfies the type soundness property and has a principal type inference algorithm. The type system therefore provides a foundation for a statically typed object-oriented language enriched with polymorphic dynamic messages.