A study of Rossia macrosoma in Portuguese waters with emphasis on its biogeographical, biological and biochemical characteristics is presented. Approximately 1000 specimens collected in 32 cruises between 1990 and 2003 were used to determine the distribution and abundance of the species and some population biology parameters (weight, length, sex and maturation). The first biochemical characterization of the species at different stages of gonad development was made. The species was found at depths up to 860 m along the whole Portuguese coast. The sex ratio was approximately 1:1 and did not vary with depth or position along the coast (P/0.05). Females grew larger (84:67 mm mantle length) and heavier (161:100 g total body weight) than males. Mature specimens of both sexes constituted B/1% of all records. Immature and maturing specimens were found year round, without significant differences between female monthly distributions (P/0.05). No significant difference between sexes in the distribution of maturity stages with depth or position along the coast was noted (P/0.05). During sexual maturation there was a significant increase (P B/0.05) in total amino acid, protein, lipid and fatty acid contents in the gonad, but the allocation of these organic compounds from digestive gland and muscle was not evident. In the three tissues analysed, the major fatty acids were 16:0, 18:0, 18:1, 20:1, 20:4n-6, 20:5n-3 and 22:6n-3, the major essential amino acids were lysine, leucine and arginine and the major non-essential amino acids were glutamic acid, aspartic acid and serine.