We investigate parametric polymorphism in message-based concurrent programming, focusingon behavioral equivalences in a typed process calculus analogous to the polymorphic lambdacalculusof Girard and Reynolds.Polymorphism constrains the power of observers by preventing them from directly manipulatingdata values whose types are abstract, leading to notions of equivalence much coarser thanthe standard untyped ones. We study the nature of these constraints through simple examplesof...