we studied the breeding biology of the rock shag (phalacrocorax magellanicus) at punta loma colony, chubut, argentina, during 2001 and 2002 breeding seasons. the egg laying period was long (3-4 months) and asynchronous, while fledging success was low (50%). during the first breeding season, the number of fledglings per nest was not related to laying date, while during the second breeding season there was a reduction in the fledging success of the pairs that laid their eggs late in the season. a high percentage of couples renested after loosing the first clutch (16-22%). in both seasons, the number of chicks fledged in replacement clutches was similar to that in first clutches. the extended and asynchronous egg-laying period, combined with a high rate of renesting (with a fledging success similar to that of single-laying couples), and the low relationship observed between breeding success and laying date, supports the idea of a highly stable and predictable food source for the rock shag.