Sargassum fulvellum (Turner) C. Agardh, an edible brown alga is farmed commercially by sexual reproduction and vegetative regeneration. Investigations were made on the phenology, abundance and maturity of reproductive structures in mature fronds, egg release and young germling development under different light conditions (20, 50, 80 and $100{mu}mol/m^2/s$) and temperatures (5, 10, 15, 20 and $25^{circ}C$). Monthly sampling was carried out by SCUBA diving at Chungsando on the southwestern coast of Korea from September 2002 to August 2003. The Maximum length of thalli was $104.6{pm}20.7{cal}cm$ in March 2003 when the water temperature was $9.0^{circ}C$ and minimum was $0.8{pm}0.5{cal}cm$ in June when the water temperature was $19.5^{circ}C$. Receptacle formation was observed from February to April. The peak period of egg release for this alga was in April when the water temperature was about $10^{circ}C$ in nature. In the culture regimes of temperature and irradiance, the egg release of the excised female receptacle was highly affected by temperature. The maximum rate of egg release was $96.7{pm}5.8{\%}$ under $20^{circ}C$ and $80{mu}mol/m^2/s$. The maximum length of young germlings was $3.9{pm}0.2{cal}mm$ after 35 days culture under $15^{circ}C$ and $80{mu}mol/m^2/s$.