The effects of several treatments for pest eradication on natural specimen DNA were examined using freeze-dried mushroom and freeze-dried muscle. The data clearly showed that the fumigants methyl bromide, methyl bromide/ethylene oxide mixed gas, ethylene oxide, propylene oxide and methyl iodide, all caused significant degradation of specimen DNA, even with a single fumigation. Subsequent steps at amplification of certain DNA fragments by PCR (polymerase chain reaction) were difficult in these samples. There were more problems in amplifying larger DNA fragments than smaller fragments. The main cause for the decreased efficiency of PCR was thought to be the degradation of template DNA. On the other hand, thermal methods (heating and low temperature), 60 percent carbon dioxide treatment, and fumigation by sulfuryl fluoride seemed scarcely to affect the DNA molecules of the specimens, allowing subsequent PCR to be successfully performed. Direct sequencing of the PCR products was also performed to check for the possibility of modifications to the DNA sequences. When the PCR products were amplified to the sufficient amount for direct sequencing, the DNA sequences were normal in our examination of a few gene regions.