Activity and distribution of I elements were analized in the 167 isofemale lines of Drosophila melanogaster sampled from Yong Jong Island population in Korea. All isofemale lines were classified to be I strain by SF sterility test. The average copy number of I elements was estimated to be 25.11 copies per haploid genome by in situ hybridization. After about 30 generations, the average activity of I elements showed the tendency to decrease 0.91 to 0.79 and the cytotype of the isofemale lines converted from I cytotype to R cytotype, while they have been maintained in laboratories. The coefficient of correlation was estimated to be 0.454 between the activity and copy number of I elements.