Reproductive and moulting biology in the portunid crab Liocarcinus depurator are described from monthly samples taken from February 1984 to January 1985 in the northwestern Mediterranean Sea. The peak moulting period in females occurred at the end of spring, when copulation took place. Ovarian maturation then began. The maximum proportion of females with fully developed ovaries was found towards the end of summer and in autumn. The main spawning period began in November and lasted until January, however small numbers of ovigerous females occurred throughout the year. Most posteedysial females had sperm plugs in the gonophores. Size at sexual maturity in females is considered to be around 18-20 mm carapace length. Number of eggs carried by ovigerous females increased with size, ranging from about 30000 to about 230000. In males, moulting and maturity seasonal patterns are not as marked as in females.