The present study addressed hypotheses about cyclical entrainment between interacting dyad members in their moment-to-moment levels of dominance and affiliation. Using a computer joystick technique, observers recorded the continuous stream of behavior for each partner in 50 mixed-sex dyads, and the data for each dyad were submitted to time-series analyses, including cross-spectral analysis. Although potentially interesting individual differences emerged, in most dyads, partners shared behavior cycles of roughly the same frequency with strongly correlated variations in amplitude (coherence). Consistent with interpersonal theory, partners' affiliation behaviors were very strongly in phase, whereas their dominance behaviors were equally strongly out of phase. In addition, these cyclical forms of interpersonal complementarity were distinguishable from other forms, such as mutual adjustment in overall levels.