The average length at which female Nephrops norvegicus attain their first maturity was determined during 1977 and 1978 by following the presence of spermatophores and development of the ovaries. During 1978, females were sexually mature at the age of 2 but in 1977 they did not mature before the age of 3. The cumulative maturity curves are sigmoid, due to an overlapping of Gaussian curves. A logistic equation was developed to calculate the percentage of mature females at each length. There is no evidence that the present fishing levels reduce the chances of a female with developing ovaries to be fertilized, since 90 to 100% of such females bear spermatophores.