Ly Son is a small but important archipelago of Vietnam. It contains different ecosystems ranging from urbanized to natural vegetation. However, little was known about bats and the general biodiversity of this archipelago. In August 2017 and May 2022, the author conducted a series of bat surveys throughout the archipelago. Bats were captured using mist nets and harp traps. Echolocation calls were recorded using the PCTape system at every netting and trapping site and inside a flight tent. Recorded calls were analyzed using the Selena software. Results from the survey confirmed that the Ly Son archipelago is home to two bat species belonging to two genera, two families: Pteropodidae (Cynopterus sphinx) and Vespertilionidae (Myotis muricola). Of these two species, C. sphinx was very common while M. muricola was rarely recorded in the archipelago. Remarkably, the echolocation signal duration of M. muricola is in a range of 2.6-5.2 milliseconds which is longer than descriptions in previous publications.