A comparison between localities currently occupied by a species and localities where the species recently underwent local extinction is a powerful way to understand the causes of population decline. Two closely related treefrog species occur in the Republic of Korea: Hyla japonica, which is distributed widely throughout northeastern Asia, and H. suweonensis, which occurs sympatrically on a narrow strip of the western coastal plains in Korea. Males of both species produce species-specific advertisement calls. To determine the spatiotemporal distributions of H. suweonensis and H. japonica during their breeding seasons, auditory monitoring was conducted in 70 localities throughout the historic range of H. suweonensis. A group of volunteers conducted the monitoring, supplemented with call recordings for validation. The endangered H. suweonensis was found in only 14 of 70 localities, designated as syntopic, whereas H. japonica occurred in all 70 localities. The calling activity of H. japonica was constantly vigorous throughout the study period, whereas the calling activity of H. suweonensis was generally weak. The analysis of binary logistic regression identified the percentage of rice-paddy field as the significant variable. The syntopic localities had much higher percentages of the rice-paddy field than the non-syntopic localities did. Localities where the rice-paddy fields are fragmented and isolated by commercial and residential complexes are not likely to harbor the H. suweonensis populations. The spatiotemporal distributions of the two treefrog species throughout the breeding seasons confirm that H. suweonensis is indeed rare and requires swift and decisive measures, including the establishment of protected areas.