During growth of the genetically engineered E. coli CU103 in different media, extracellular DNAs released from the cells were studied. The extracellular DNAs released in the medium were concentrated by an thanol precipitation method and then quantified by a fluorescence method using Hoechst 33258. The released extracellular DNAs were also examined by gel electrophoresis and identified by Southern hybridization for the cloned pcbCD genes. The chromosomal DNAs and recombinant plasmid containing the cloned genes were observed to be released in an exponential growth phase. In Luria-Bertani (LB) broth and MM2-GLUCOSE, 210 and 69 ng/ml of DNAs were detected, respectively, after 3-4 days incubation at $30^{circ}C$ and at pH 7.0. But the released DNAs were measured to be about 10-15 ng/ml in filtered river water (FW) and Tris-EDTA (TE). The at both $15^{circ}C$ and $4^{circ}C$, but the released DNAs were more easily degraded at the higher temperature. The extracellular DNAs were produced about 2 times more at pH 7.0 than at both pH 5.0 and pH 9.0 in MM2-glucose medium at $30^{circ}C$. Therefore, the extracellular DNAs were found to be released actively from the cells during growth in liquid media.