I investigated an effective way to generate a metagenomic library from DNA prepared from environental samples. The sizes of DNA extracted from environmental samples were usually in the range of 10 to 100 kbp as estimated from $0.4\%$ agarose gel electrophoresis. Because of this small size, a fosmid, rather than BAC, was chosen as a vector. It was found that, for the successful generation of metagenomic library, the selection of DNA with the sized of about 40 kbp was critical and, therefore, a simple agarose gel electrophoresis system was developed to select this size of DNA. By the procedure described in this report, I obtained metagenomic libraries containing 25,000 fosmid clones, which corresponded to 1,000 Mb of metagenomic DNA