Increased imports of genetically modified (GM) soybean and maize might cause genetic contamination of those crops that are conventionally bred, as well as wild soybeans within Korea. Leaves of maize and both cultivated and wild soybeans were sampled in and near rural fields to detect the presence of transgenes. Roadsides around a major grain port in Incheon were also surveyed to monitor the occurrence of incoming GM soybean and maize. The amplificability of DNA extracted from the collected samples was determined by PCR using soybean- or maize-specific primers: lectin and zein genes, respectively. The presence or absence of transgenes was detected by primer sets for the 35S and nos genes. Transgenes were not found in the cultivated or wild soybean or in the maize collected from cultivated fields. However, we obtained one GM maize plant among seven along the roadsides around Incheon Port. Although the effect of a single GM maize plant would be negligible and would not pose any threat to natural environments, an increase in the import of GM plants might lead to future, unapproved cultivation of GM crops. Therefore, appropriate monitoring is necessary to detect the occurrence of GM plants in areas around grain receiving ports and within agroecosystems.