This paper examines the early career of CHO Pok Sung(趙福成), one of the representative Korean naturalists in Colonial Korea. The point is that Cho could develop his career as a naturalist in connection with Japanese imperial expansion. Cho was trained as an apprentice by MORI Tamezo(森爲三) and DOI Hironobu(土居寬暢), and became an assistant for the preparatory course of biology in Keijo Imperial University(京城帝國大學) in 1930. From the collaboration with these Japanese naturalists, Cho extended his research from the insects of Korean peninsula to those of Manchuria and China. Cho could also extend his personal network to Japanese-occupied territories in China.
Both Koreans and the Japanese government regarded Cho as a competent naturalist. Cho participated in the Association for the Diffusion of Scientific Knowledge(科學知識普及會), which included Korean elites from various levels of society. In 1942, Cho, the former local apprentice of Japanese naturalists, could be promoted as high as a researcher in Nanjing Museum(南京博物院). The appreciation of Koreans and the Japanese government on Cho shows the border between cross-national collaboration and nationalism was not clear in the field of science in Colonial Korea. The case of Cho illustrates how Korean naturalists practiced in the research environment of Colonial Korea.