As a model for plant-type chromosomes, we have been characterizing molecular organization of Chlorella vulgaris (C-169) chromosome I. To locate structurally important chromosome regions, bent DNA fragments of this chromosome were systematically obtained and analyzed. By two-dimensional polyacrylamide gel electrophoresis, 150 clones containing bent DNA fragments were screened from C. vulgaris genomic libraries. These clones were classified into 3 groups by Southern hybridization. A group that hybridized with all Chlorella chromosomes contained fragments carrying a retroposon-like element. A clone (p2DS-66A) derived from chromosome VI strongly hybridized with chromosome I as well as the others. The corresponding chromosome I sequences (pI-21 and pI-31) were analyzed in detail.