The present study was undertaken to cast some light on this methodological problem as well as to determine how representative the materials used in biological research on Illex argentinus have been so far. It is contended that trawling, as a forced sampling technique, provides a sample representative of an Illex population, while jigging, as a harvesting/sampling method based on behavioural responses of the squids, is selective. The squids were obtained during commercial trawling and jigging operations undertaken on 4-15, 24-25 and 28-31 March and 2, 7-11 April 1992 in the Illex-rich Falkland Interim Conservation Zone. Differences among basic biological parameters of the population (e.g. length and maturity frequency) indicate that jigger-caught Illex were larger than those caught by trawling by 7 mm (males) and 12 mm (females) on the average. Jiggers affected the behavioural responses of the squids and resulted in higher frequencies of larger individuals. Those frequencies being higher by: 13.5% in 25-28 cm long males, the general range being 19-28 cm; 21.2% in 26-30 cm long females, the general range being 16-30 cm. Differences between the jigger-and trawl-caught squid lengths can be described with a linear regression. The jigger-caught Illex were more mature than those caught with the trawl by 0.8% (males) and 5.1% (females), on the average.