Auditory forms of nonpersonal communication have rarely been evaluated in informal settings like parks and museums. This study evaluated the effect of an interpretive audio tour on visitor knowledge and social behavior at Carlsbad Caverns National Park. A cross-sectional pretest/posttest quasi-experimental design compared the responses of audio tour users (n = 123) and nonusers (n = 131) on several knowledge questions. Observations (n = 700) conducted at seven sites within the caverns documented sign reading, time spent listening to the audio, within group conversation, and other social behaviors for a different sample of visitors. Pretested tour users and nonusers did not differ in visitor characteristics, knowledge, or attitude variables, suggesting the two populations were similar. On a 12-item knowledge quiz, tour users' scores increased from 5.7 to 10.3, and nonusers' scores increased from 6.2 to 8.4. Most visitors were able to identify some of the park's major messages when presented with a multiple-choice question, but more audio users than nonusers identified resource preservation as a primary message in an open-ended question. Based on observations, audio tour users and nonusers did not differ substantially in their interactions with other members of their group or in their reading of interpretive signs in the cave. Audio tour users had positive reactions to the tour, and these reactions, coupled with the positive learning outcomes and negligible effects on social interaction, suggest that audio tours can be an effective communication medium in informal educational settings.