Acoustic signals used during the pair-forming behavior of Isophya modesta modesta are described for the first time. Specimens from Hungary, Rumania and the Ukraine have been studied. The male calling song of I. m. modesta is composed of single syllables. Each syllable consists of a shorter main impulse-series (120 to 240 ms) and a longer terminal impulse-series (200 to 2500 ms), separated from each other by a silent interval of 4 to 9 s. Uniquely in I. m. modesta, females can produce 2 response songs during 1 syllable of the male: after the male's main impulse-series, and most frequently, after the terminal impulse-series. Playback experiments revealed that the second female response song is related to the end of the male's terminal impulse-series. I. m. modesta and I. m. rossica were formerly regarded as two different species only subtle morphological differences being known between the 2 taxa. But as the male calling songs are apparently identical, and with the present state of our knowledge, it is reasonable to treat “I. rossica” as a subspecies of I. modesta. The only remarkable difference in the pair-forming acoustic communication of the 2 subspecies is the different timing of female response song, indicating that the 2 taxa have diverged from each other to a certain degree.