Audio computer-assisted self-interviewing (audio-CASI or ACASI) has been widely adopted around the world as a method for eliciting more candid responses to sensitive questions in surveys. While few studies have explored the added advantages audio may bring over text-CASI, those that have (e.g., Tourangeau and Smith, 1996, Public Opinion Quarterly 60(2):275–304) found large effects for self administration over interviewer administration, but only modest additional gains from the audio enhancement. In this paper, we explore the use of audio-CASI versus text-CASI in a national survey of fertility-related issues in the United States (the National Survey of Family Growth). In the pretest, male and female respondents were randomly assigned to audio-CASI (n = 299) or text-CASI (n = 312). We compare the distributions of substantive responses between modes and examine a variety of paradata (e.g., keystroke files, time stamps) to examine the use of the CASI instruments. The main study, which interviewed 7,643 women and 4,928 men aged 15–44 in 2002–03, used audio-CASI only, but again we have a variety of paradata and interviewer debriefing items to examine the extent to which subjects made use of the audio enhancements to CASI. Our results indicate that most respondents make limited use of the audio features of audio-CASI and accordingly the gains produced by this technology are modest relative to text-CASI.