The study of acoustic communication in lizards has been restricted and mainly focused on Gekkota. The rest of the lizards, Unidentata, are generally considered voiceless, although there are sparse reports on vocal sound production in members of different families. We analyzed the spectro-temporal characteristics of the distress calls emitted by the Unidentata lizard, Liolaemus chiliensis (the weeping lizard; Liolaemidae), the only species of this highly diverse genus (>220 species) that vocalizes. We also explored the relationships of the call characteristics with the sex and size of the individuals. The vocalizations of L. chiliensis are highly diverse and complex compared to any known call produced by Unidentata lizards. They exhibit pronounced frequency modulations, various nonlinear phenomena, and harmonics that extend into the ultrasonic range. Fundamental frequency and call duration are correlated with body size, and males emit louder but simpler calls (i.e., with less nonlinear phenomena) than females. Based on the characteristics of the vocalizations, we discuss their potential role for startling predators and/or alerting conspecifics to predation risk.