Based on size at maturity and standardized residuals from mantle length (ML)-total weight (TW) and mantle length-gonad weight (GW) relationships, this paper describes the interannual variability of the size at maturity (SM), somatic and reproductive investment of mature males and females of Dosidicus gigas during the spawning peak (October-March) for the period 1994-2006 in the Northern Humboldt Current Ecosystem (Peruvian waters). Significant changes occurred in the SM in females as well in males: before 2001 the average did not exceed 47. cm ML, while from 2001 onwards the average MS varied between 62 and 96. cm ML. The average SM was generally higher in females. There was a great individual variability of somatic and reproductive investment, being higher in large-sized mature squids. No relationship was found between SM and somatic condition. The presence of small-sized mature squids in good somatic condition under different scenarios of sea surface temperature (normal, warm or cold periods; El Niño and La Niña events) does not indicate a relationship between somatic condition and SST. The relationship between reproductive and somatic investment was variable and independent of the sex. It also varied among different years. This would indicate that females and males may respond differently under the same environmental conditions. Considering the relationship between fecundity and ML, mature females at larger sizes had a lower reproductive condition, but also a higher potential fecundity. Lower fertility potential for some small mature females is offset by greater investment in reproduction, particularly under adverse environmental conditions. The positive correlation between the abundance (CPUE) and SM shows that the biomass was greater in periods dominated by the presence of large-sized mature squid. © 2010 Elsevier B.V.