Researchers commonly use auditory surveys to determine the presence of anuran species (Bridges and Dorcas 2000, Crouch and Paton 2002, Schmidt 2003, Pierce and Gutzweiller 2004, U.S. Geological Survey 2005). Researchers conduct the surveys during the breeding season when the species of interest are vocalizing to attract mates. Researchers record environmental conditions and listen for the call of interest. In one such method, trained observers use auditory cues alone to detect and monitor the presence or absence of endangered Houston toads (Bufo houstonensis; Forstner and Swannack 2004). The Houston toad is endemic to eastcentral Texas, USA, and was first listed as an endangered species in 1970. Observers in this program train for several seasons in detection and differentiation of calls, and they must have certification from the United States Fish and Wildlife Service (USFWS). However, one problem with any in situ sampling method is that detection of organisms is imperfect (Kery 2002, MacKenzie et al. 2002, Schmidt 2003, Bailey et al. 2004). A conclusion of absence may simply be an artifact of the survey scheme, rather than actual absence. The USFWS provides explicit guidelines for audio call surveys to federally permitted individuals. The guidelines delineate the appropriate season, weather conditions, duration, and number of surveys required in a season. One of the specified guidelines is the completion of, at minimum, 6 survey nights during suitable season and weather conditions. The ability to detect Houston toads reliably with this survey protocol is a concern of the USFWS and necessary for effective management of this species. The number of auditory surveys needed to determine the presence or absence of a single species, the Houston toad, accurately is the focus of this study. This is a pressing concern when monitoring an endangered species such as the Houston toad. We estimated the probability of detecting Houston toads when present (detection probability) and used these probabilities to determine the number of surveys required to be confident in a verdict of Houston toad absence from potential breeding sites. Study Area