This article deals with the phenomenon of overregularization in a language already extremely regular, i.e. Esperanto, in children who are learning it as their mother tongue together with one or two other national languages. It consists of an analysis of the diaries kept by Esperanto-speaking parents, tracing the development of five children who were brought up speaking Esperanto as one of their two or three mother-tongues. The children were all of European origin, and their ages ranged from one to five years.