Arbacia dufresnii is a common and abundant sea urchin in Argentinean waters. The gonad index cycle was studied for this species in Golfo Nuevo, Patagonia, from May 2001 to March 2003. Comparative studies of gonad index and gonad histology were performed for the three years of 2001, 2002 and 2003. The gonad index reflects gonadal stages, and the results showed two spawning peaks for the studied period. The first spawning event occurred during spring and a second major spawning occurred in summer. Proliferation and differentiation of gametes, and the presence of mature gonads were synchronous among males and females. Male and female individuals spawned synchronically. Gonad resorption was observed for males and females. Histological examination data reflected gonad index analysis, providing together a good estimation of the reproductive cycle of A. dufresnii. Statistical analyses showed a positive correlation between gonad index and photoperiod. The mean fresh oocyte diameter observed was 87.1 µm (SD = 1.3 µm) with a jelly coat of 12.7 µm (SD = 0.9 µm), which corresponds to planktotrophic larval development.