The Mexican tetra Astyanax mexicanus —with a native range from central/southern Mexico reaching up to the lower Rio Grande and Nueces rivers—inhabits both cave and surface environments. Life in these contrasting habitats has influenced developmental morphology, allowing for two primary morphologies to exist: cave fish and surface fish that diverged 0.2 to 1 million years ago. Cave fish are characterized by troglomorphic phenotypes: vestigial eyes, reduced pigmentation, and increased olfactory and lateral line sensitivity. Previous research indicated differences in hearing thresholds between cave and surface populations. Surface A. mexicanus were introduced to the San Antonio River, Texas, in the early 1900s, and subsequently colonized subterranean habitats in the Edwards Aquifer. These populations display evidence of divergence and rapid phenotypic and behavioral evolution, allowing observation on how sensory systems adapt to new environments in real time. Two surface and three cave fish populations were tested and compared for auditory sensitivity to sound pressure and particle acceleration, with both ecotypes responding to 4 kHz sound frequencies. Sound pressure thresholds between cave and surface populations differed significantly at one frequency (0.5 kHz), with significant particle acceleration threshold differences between 0.4 and 0.5 kHz and above 1.5 kHz. We hypothesize that the differences in frequency sensitivity may be due to different soundscapes between the cave and surface environments.