This study was concerned with the effects of child controllability on adult assertiveness, as moderated by adult control perceptions. 4 boys (aged 7--9), trained to be responsive or unresponsive, interacted in adult-child dyads on a toy construction task with 32 women. As predicted, adult locus of control moderated the effects of child behavior. Child unresponsiveness produced an effect on adult behavior only among adults with low control perceptions ("externals"). With unresponsive children, external adults had relatively less assertive voice intonation during their statements of verbal approval and disapproval than during neutral statements (typically consisting of task directions). This pattern replicated previous observation of the communication of mothers with their own unresponsive children. For all other adult-child groupings, voice assertion was higher during affective messages than during neutral messages.