In proficient users of language, left hemisphere regions mediate linguistic processing. Recent evidence has shown that the activation of these neural regions is largely independent of the modality in which a language is expressed. Natural human language processing typically engages the left perisylvian regions including left inferior frontal, middle and posterior temporal lobes. However, the scope and degree of this specialization remains to be determined. The present study examines the neural processing of a naturally developed whistled speech surrogate of Spanish, Silbo Gomero. Silbo Gomero is a form of whistled-speech used on the island of La Gomera, Canary Islands, Spain. It was developed and is used by rural shepherds for communication over long distances of difficult terrain. It is a language substitute of Spanish, where the chord tone is replaced by a loud whistle. This study indicates that a whistled signal (Silbo Gomero) whose formant composition and transitions dutifully map to extant phonemic categories in Spanish activates left posterior temporal and inferior frontal regions in persons familiar with the use of this speech surrogate. Many of these same cortical regions were found to be active when listeners comprehended Spanish sentences. For subjects unfamiliar with Silbo, language regions are not activated. Our results provided further evidence for the flexibility of the human capacity for language to process a wide variety of signal forms. The finding that left hemisphere language regions are engaged in the processing of whistled speech suggests these regions are uniquely adapted for communicative purposes, independent of signaling modality.