Infants often protest the activities of their caregivers, and this particular social interaction may provide an important window on early communication and its development. This study used naturalistic methods to investigate the development of vocal protests. Fifteen mother-infant dyads at each of 5 ages, from 3 to 18 months, were observed at home. Maternal behaviors of caregiving and prohibiting were tallied from videotapes, as were infants' protests of these behaviors. Maternal caregiving decreased with age, but maternal prohibitions increased. There were no changes over age in the probability of protesting maternal caregiving behavior; however, 12-month-olds were more likely to protest prohibitions than 6- or 8-month-olds. Older infants were also more likely to use intense protests, such as screams, than younger infants. These age-related changes were mirrored by the differences in prohibitions and protests observed between 8-month-olds who could crawl and those who could not. Findings from this study were related to previous research on infant crying as an important part of the prelinguistic communication system. (Contains 2 figures and 2 tables.)