WWeight to mantle length relationships were examined for Illex coindetii from the northern Tyrrhenian Sea in 1987. Annual and seasonal relationships showed that males were significantly heavier than females at any given length and in spring they were relatively heavier than in summer. Length-weight relationships examined in relation to sexual maturation revealed a change once the maturation process starts. Immature individuals exhibited negative allometric growth (b<3) in both sexes while in maturing and mature males growth was positively allometric (b>3) and in females isometric (b=3). The length-somatic weight relationships computed by sex and maturity stage indicated that in maturing and mature females the reproductive organs grow at a relatively faster rate than the rest of the soma.