For studying growth of sea squirt, Halocynthia roretzi, from laboratoy grown seeds, in situ culture experiment was carried out off Hansando located in the southern part of Korea from February 1984 to July 1986. In the growth of the mean body height, sea squirt grew up to 9.15 cm during the above period after they were seeded at about 0.05 cm. The meat weight of 1.27 g at the beginning increased to 41.69 g from February 1985 to March 1986. The decreasing tendency in body height, body breadth, total weight and meat weight of sea squirt was observed during the summer season in 1985 when the water temperature reached $23.53^{circ}C$ the maximum value in monthly mean, and the specific gravity of sea water remained at the lowest level of 1.0233 in monthly mean. Sea squirt became gradually elongated with the decreasing tendency in the relative growth ratio of body breadth to body height with aging. The monthly mortality of sea squirt was especially higher in June and July than in the other months, and the average monthly mortality was $10.83\%$.