The ecological characteristics of Pseudoblennius cottoides were investigated off Dolsan Island, Yeosu, Korea, from June to December, 2001. Their habitats were the reef zone and surrounding embankment which was covered with seaweeds. Their range of total length (TL) was 6.10-8.40 cm (n=15), and the individuals (over 8.10 cm) of them accounted for in June. It was 8.60-15.7 cm (n=67), and the individuals 11.1-12.0 cm in TL accounted for in October, 2001. The female to male sex ratio was 1:0.48 (n=185). The gonads were in the shape of a rod and the ovaries were large in size. In June, the histology of the seminiferous tubules was observed. In August, the testicles appeared. In October, spermatocyte was observed in the testes, and ovaries were observed in relation to circumference popularity and oocyte.