The alarm vocalizations of the whistling rats Parotomys brantsii and P. littledalei were investigated at the Goegap Nature Reserve in the Northern Cape, South Africa, where they occur sympatrically. Parotomys brantsii's call is a single note vocalization, characterized by an upward frequency sweep and high frequency plateau with a dominant frequency of 10.0 ± 0.3 kHz and duration of 164 ± 11 ms. The alarm whistle of P. littledalei has three overlapping components and is both shorter (53 ± 5 ms) and lower in dominant frequency (7.7 ± 0.1 kHz) than that of P. brantsii. The frequency bandwidth of P. littledalei calls (10.2 ± 0.7 kHz) is significantly higher than that of P. brantsii (6.8 ± 0.4 kHz). These significant distinctions are attributed to habitat preferences of the two species. Our data support the acoustic adaptation hypothesis in that P. littledalei, which inhabits a more closed habitat, has calls which are lower in frequency than P. brantsii calls, but contrary to the hypothesis, P. brantsii calls do not show greater frequency modulation than those of P. littledalei. Despite these differences, the alarm calls of the two species are both high-pitched, and may have converged in structure to be difficult for predators to locate.