Ecological characteristics of a brown alga, Scytosiphon lomentaria, were investigated from January 2021 to December 2021 in its natural habitat off Sodol, Jumunjin, eastern coast of Korea. The S. lomentaria population at the site formed widespread patches on mid shore. During the investigation, environmental conditions including seawater temperature, salinity, and dissolved oxygen were monitored at the site. Growth and maturation of the S. lomentaria population were identified through qualitative and quantitative investigations. An estimation of the effective cumulative temperature for maturation of the alga was obtained based on growth data and a biological zero temperature of 8°C. Sporangia were observed from February to May when seawater temperatures ranged from 7.7°C to 16.4°C. A maturation peak was detected in April when seawater temperature was 12.1°C. After zoospore release, the alga became bleached and only the crust remained after June. Developmental initiation of the thallus occurred at temperatures above 8°C. Its maturation required approximately 162 degree-days.