A study of the breeding biology of the Great Cormorant (Phalacrocorax carbo sinensis) was carried out during 1993-1998 in die Po River Delta, a major wintering area in northeastern Italy. The first of five colonies was founded in 1993, and breeders increased from 69 pairs in 1994 to 285 pairs in 1998. The period of egg-laying lasted from early March to late July, but 65% of all clutches were initiated between late March and the end of April. The average clutch size (+/-SD) was 3.83 +/- 0.65 eggs (range two to six). The survival rate of entire nests, evaluated by the Mayfield method, was always higher during the nestling stage than during incubation. The majority of losses occurred during the egg stage due to predation by Hooded Crows (Corvus corone cornix). During the five years, hatching success was 72-91% and total nest success, 67-86%. The number of young Fledged per successful nest ranged between 2.3 and 2.9. The reproductive parameters of Great Cormorants recorded in the Po Delta lay at the upper range reported for NW Europe populations and are close to the estimated values for newly-established colonies in Italy and for growing colonies in the traditional breeding areas. Unlike other Italian colonies, the availability of breeding sites and food presently do not limit expansion of the breeding population in the Po Delta. If no control measures are undertaken, it is likely that this southern population will continue to grow.