Phou Khao Khouay leaf-nosed bat (Hipposideros khaokhouayensis) is aglobally rare and vulnerable species of the family Hipposideridae. It has been knownonly from the type locality in Laos and two localities in Vietnam. Its echolocationcalls were included in several publications but still poorly understood. BetweenNovember 2023 and April 2024, the authors conducted three field surveys in LangSon province and Hai Phong city, northeastern Vietnam, in order to seek furtherunderstanding of its potential distribution and echolocation behavior. Twelve adultindividuals of the species were captured using harp traps and mist nets foridentification in the field based on their morphological diagnoses to confirm thetarget species' identity. The PCTape system was employed to record echolocationcalls while bats were emerging from caves. Seven sound parameters (CF frequency,CF duration, tFM frequency, tFM bandwidth, tFM duration, pulse duration, andinter-pulse interval) of the recorded calls were analyzed and measured using theSelena software. Their values are in a range of 90.2 - 91.1 kHz, 3.1 - 8.9 ms,75.6 - 84.5 kHz, 6.1 - 15.0 kHz, 0.9 - 1.7 ms, 4.5 - 10.1 ms and 15.1 - 31.4 ms,respectively. The present study results are considerably different from thedescriptions in previous publications. This paper also provides the first data on theinter-pulse interval of the species echolocation calls and a methodological noteon this sound parameter.Phou Khao Khouay leaf-nosed bat (Hipposideros khaokhouayensis) is aglobally rare and vulnerable species of the family Hipposideridae. It has been knownonly from the type locality in Laos and two localities in Vietnam. Its echolocationcalls were included in several publications but still poorly understood. BetweenNovember 2023 and April 2024, the authors conducted three field surveys in LangSon province and Hai Phong city, northeastern Vietnam, in order to seek furtherunderstanding of its potential distribution and echolocation behavior. Twelve adultindividuals of the species were captured using harp traps and mist nets foridentification in the field based on their morphological diagnoses to confirm thetarget species' identity. The PCTape system was employed to record echolocationcalls while bats were emerging from caves. Seven sound parameters (CF frequency,CF duration, tFM frequency, tFM bandwidth, tFM duration, pulse duration, andinter-pulse interval) of the recorded calls were analyzed and measured using theSelena software. Their values are in a range of 90.2 - 91.1 kHz, 3.1 - 8.9 ms,75.6 - 84.5 kHz, 6.1 - 15.0 kHz, 0.9 - 1.7 ms, 4.5 - 10.1 ms and 15.1 - 31.4 ms,respectively. The present study results are considerably different from thedescriptions in previous publications. This paper also provides the first data on theinter-pulse interval of the species echolocation calls and a methodological noteon this sound parameter.