This study relates to the size at morphological and physiological sexual maturity of the deep sea crab Anamathia rissoana. Between August 1997 to July 2000, 469 specimens were collected at depths of between 400 and 620 m off the western coast of Sardinia (Mediterranean Sea). On the basis of cheliped morphology, the males were classified into three demographic categories: immature, adolescent, or adult. In addition, a discriminant function analysis was performed in order to classify the males (post hoc prediction) using the biometric data relating to the four distal segments of the endopod of the right cheliped. For the females, only two demographic categories were detected: juvenile females with flat abdomen and adult females with domed abdomen. Morphological maturity was estimated from the allometric relationships between carapace width and propodus length in males or maximum abdomen width in females. For both sexes, allometric relationships were established in the specimens of each of the demographic categories. Physiological maturity was determined by observing testes or ovary size, colour, morphology, and development. Functional maturity was presumed to be the minimum size at which each sex was morphologically and physiologically able to reproduce. In males, the size at physiological maturity was clearly earlier than the size at morphological maturity, unlike in females where such values were similar.