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Summary 1. Lake loannina (Greece) is occupied by three sympatric water‐frog phenotypes, which by bioacoustic and morphological criteria represent R. ridibunda, a new species of Rana, and hybrids of these two species. 2. The unnamed water‐frog species receives the name Rana epeirotica sp. n., Epeirus frog, after the site in the Epeirus region at which it was collected. 3. The mating calls are specific and hence are a very reliable discriminator of these water frogs. The mating call of R. epeirotica consists of a series of very brief pulses produced at relatively long intervals. The call therefore has a rattling sound. When the temperature rises the call duration and the pauses between the calls decrease, while the number of pulses per call and the pulse repetition rate increase. The mating call of R. ridibunda consists of pulse groups. When the water temperature rises the pulse groups and the intervals between the groups become shorter; the result is a reduction in call duration, because the number of pulse groups per call does not change. The mating call of the hybrids resembles that of R. esculenta. Neither the number of pulse groups per call nor the number of pulses per group changes with temperature, but the temperature does affect the call duration as well as the intervals between the pulse groups and between the calls. 4. R. ridibunda and R. epeirotica differ from one another in a number of morphological parameters, and the hybrids are intermediate between the two. In R. ridibunda the callus intemus is appreciably larger than in R. epeirotica. The most important character ratios by which the three phenotypes can be distinguished from one another are body length/tibia length, body length/length of callus intemus, and digitus primus length/length of callus intemus. 5. During the reproductive period the color and markings of the animals are additional important aids to identification. The tympana are brown in R. ridibunda, green in R. epeirotica and bronze‐colored in the hybrids. The coloration of the back and the patterns on the thighs are also characteristic. As in R. ridibunda, the males of R. epeirotica and of the hybrids have gray to black vocal sacs. Zusammenfassung Bio‐akustische und morphometrische Untersuchungen an Wasserfröschen (Gattung Rana) des loannina‐Sees in Griechenland und Beschreibung einer neuen Art (Anura, Amphibia) 1. Im See bei Ioannina (Griechenland) kommen drei sympatrische Wasserfrosch‐Phänotypen vor, die bio‐akustisch und morphologisch R. ridibunda, eine neue Wasserfrosch‐Art und Hybriden dieser beiden Arten repräsentieren. 2. Die unbenannte Wasserfrosch‐Art erhält den Namen Rana epeirotica sp. n., Epirus‐Frosch, nachdem der Fundort im Epirus liegt. 3. Die Paarungsrufe sind spezifisch und somit ein sehr sicheres Hilfsmittel zur Unterscheidung dieser Wasserfrösche. Der Paarungsruf von R. epeirotica ist aus sehr kurzen Impulsen aufgebaut, die in verhältnismäßig langsamer Folge abgegeben werden. Die Rufe klingen daher schnarrend. Bei Temperaturanstieg nehmen die Rufdauer und die Pausen z...

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