Frogs and toads exhibit complex vocal behavior, often in the context of mating and resource defense. Although over 6,500 species of frogs and toads have been described, detailed accounts of vocal behavior exist for very few species. In this study, we provide the first detailed investigation of the vocal behavior of Neotropical Yellow Toads, Incilius luetkenii, based on recordings collected from animals in northwestern Costa Rica. We provide quantitative measurements of two call types, we test the hypothesis that male body size is related to the structure of male calls, we investigate the context in which males produce calls, and we discuss the importance of their vocal behavior in the framework of an aggressive scramble competition mating system. We found that male I. luetkenii produce two vocalizations during their explosive breeding aggregations: advertisement calls and release calls. Advertisement calls are 2.5 sec-long vocalizations consisting of a series of pulses. Release calls are 0.6 sec-long vocalizations consisting of a small number of pulses and produced when other males attempt amplexus on them. As predicted, spectral properties of advertisement calls were inversely related to body size. Video analysis of male calling behavior revealed a call rate of 0.58 calls per minute and revealed that most advertisement calls occur within 2 sec following a contact with another male. Our findings suggest that advertisement calls and, by extension, chorusing behavior are associated with aggression between males during mating aggregations and that males may communicate acoustic information about their body size during male–male encounters.