NOVELTY - A nucleic acid fragment (I) comprising 18-23 nucleotides capable of hybridizing to 16SrRNA or rDNA of a bacterium capable of degrading chlorinated ethane and having a base sequence complementary to a sequence chosen from SEQ ID No. 1-17 and a base sequence having 90% or more homology with any one of SEQ ID No. 1-17, is new.    USE - (I) is useful for detecting chlorinated ethane degrading bacteria, which involves performing PCR by using the nucleic acid contained in the sample as template and (I) as primer, and detecting the produced DNA fragment. (II) is also useful for detecting chlorinated ethane degrading bacteria, which involves contacting sample or nucleic acid from the sample with (II), performing RNA or DNA hybridization and detecting the bacteria by using the label as a parameter (claimed).    ADVANTAGE - (I) enables highly precise easy, rapid and efficient detection of bacteria having biodegradability of chlorinated ethane (e.g. 1,1,1-trichloroethane and 1,1,1-dichloroethane) such as Dehalobacter restrictus.    DETAILED DESCRIPTION - A nucleic acid fragment (I) comprising (a) 18-23 nucleotides capable of hybridizing to 16SrRNA or rDNA of a bacterium capable of degrading chlorinated ethane and having a base sequence complementary to a sequence chosen from SEQ ID No. 1-17 (KWI-Deb 1-17) and a base sequence having 90% or more homology with any one of SEQ ID No. 1-17, or (b) 10-15 nucleotides capable of hybridizing to 16SrRNA or rDNA of a bacterium capable of degrading chlorinated ethane and having a base sequence complementary to a base sequence comprising 10 or more contiguous bases of any one of SEQ ID No. 1-17. tgaagcttgcgatttagc (SEQ ID No. 1), tagcgcttagggatggac (SEQ ID No. 2), ggaagaacggcatctgtg (SEQ ID No. 3), gcttaactccgtaaagca (SEQ ID No. 4), gttagggaagaacggcatctgtg (SEQ ID No. 5), aaataatgcaggtgattgacggt (SEQ ID No. 6), ggagatagttaatgaagctt (SEQ ID No. 7), gcgatttagctatcttagtg (SEQ ID No. 8), ttgtccgagaggcatctctt (SEQ ID No. 9), ggagaagaaagctggcctct (SEQ ID No. 10), gaaaatgctagcgcttaggg (SEQ ID No. 11), aagaacggcatctgtgtaaa (SEQ ID No. 12), taatgcaggtgattgacggt (SEQ ID No. 13), ggcaatataagtctgatgtg (SEQ ID No. 14), gtaaagcattggaaactgta (SEQ ID No. 15), aggcttgacatccaactaat (SEQ ID No. 16), cccttcggggaaagttgaga (SEQ ID No. 17). INDEPENDENT CLAIMS are included for the following:    (1) probe (II) for detecting chlorinated ethane degrading bacteria, comprising (I) labeled by a labeling substance;    (2) degrading chlorinated ethane, involves detecting chlorinated ethane degrading bacteria in underground water or soil sample by using (I) or (II), introducing culture solution of the detected bacteria into underground water or soil contaminated with chlorinated ethane; and    (3) monitoring chlorinated ethane degrading bacteria, involves detecting the bacteria using (I) or (II), determining the number of bacteria in the detected bacteria contained in the chlorinated ethane contaminated underground water or soil or underground water or soil introduced with the bacteria.