Females of the Norway lobster Nephrops norvegicus (L.) incubate their eggs under the abdomen for a period of several months. The authors estimated the individual fecundity of N. norvegicus (1) before spawning, and (2) at the end of the incubation period. From these two relationships, they then calculated the average egg loss. Egg loss seems to be relatively higher for small females than for large ones. The mean egg loss estimated for the Bay of Biscay (45%) differs from the only other existing value (75%) calculated by other authors for Portuguese waters. Egg loss may regulate stock-density, and may be a mechanism intervening in stock-recruitment.