Résumé
The derivative D of a word is obtained by writing the number of consecutive equal letters when the word is spelled from left to right. For example, D(1 1 2 3 3) = 2 1 1 2 2 3 (two 1, one 2, two 3). We start the study of the behaviour of binary words generated by morphisms under the D operator, focusing in particular on the Fibonacci word.