In vitro selection or systematic evolution of ligand by exponential enrichment (SELEX) has been devised for the identification of high-affinity oligonucleotide aptamers to target molecules. However, the selection process is repetitive and time-consuming. We have developed an automatic for in vitro selection by assembling an affinity chromato-column, a PCR thermal cycler, a HPLC and a sample operation system. Several molecular biology methods were optimized for the machine. Automated selection was used to generate nucleic acid aptamers interacting specifically with an environmental contaminant.