The reproductive biology of the marbled goby, Pomatoschistus marmoratus (Risso, 1810), was studied in the Strymon River estuarine system (northern Greece) between September 1997 and August 1999. Samplings were conducted on a monthly basis at the mouth of the river using a bag seine net and overall a total of 4 563 individuals were collected. The total length of males ranged between 2.5 and 6.0 cm and that of females between 2.8 and 5.7 cm, while that of unsexed individuals ranged between 1.2 and 2.9 cm. Sex ratio was 1.54:1 in favour of the females and statistically different from unity. The spawning of the species was extended, occurring between February and May. Mean absolute fecundity (F) was 1 386 (SE=8) oocytes and showed a significant positive exponential relationship with total length (F=14.387TL2.92), and total weight (F=1351TW0.83) of the fish. The relative fecundity ranged between 878 and 3 444 oocytes/g of total weight. Mean size at first maturity was estimated at 3.82 cm for the females and 4.66 cm for the males.