In crustaceans, size at first maturity is often determined by secondary sexual characters. The present work tries to determine the size at first maturity in Nephrops norvegicus from South-Brittany area with the development of some secondary sexual characters. We study a qualitative character - the presence or absence of ovigerous setae in females and biométrie characteristics - the width of the second abdominal segment in females, the length of the propod at the first pereiopod in males and females - The study of the relative growth of the propod length shows that males may reach sexual maturity at 19mm carapace length. The first maturity in females is reached at 23.5mm later than the coming out of the ovigerous setae. That carapace length is obtained through the study of the relative growth of the abdominal length.