This study examines the seasonal variability in gillnet size selectivity for the main Chilean hake (Merluccius gayi gayi) fishery off the coast of central Chile. Selectivity estimates were based on five gillnet mesh sizes (50, 60, 70, 80, 90 mm) in four traditional fishing areas, and during periods of low and high reproductive activity. Average fish length caught in the earlier period (low reproductive activity) was 30.8 cm LT (±4.17), and significantly longer (p <.05) at 35.9 cm TL (±6.15) in the latter period (high reproductive activity). For males, the selectivity factor was similar in both periods, thus the estimated modal lengths did not differ seasonally (p >.05). For females, however, both the selectivity factor and the estimated modal lengths changed significantly depending on the time of year (p <.05). A significant increment was estimated in the girth/perimeter ratio and condition factor during the period of higher reproductive activity. This change in the fish girth of females during spawning helps explain the significant reduction in their selectivity factor estimates in August–October.