Extending first-order logic with ML-style polymorphism allows to define generic axioms dealing with several sorts. Until recently, most automated theorem provers relied on preprocess encodings into mono/many-sorted logic to reason within such theories. In this paper, we discuss the implementation of polymorphism into the first-order tableau-based automated theorem prover Zenon. This implementation led us to modify some basic parts of the code, from the representation of expressions to the proof-search algorithm.