Researchers have demonstrated that a person's rhythmic movements can become unintentionally entrained to another person's rhythmic movements or an environmental event. There are indications, however, that in both cases the likelihood of entrainment depends on the difference between the uncoupled periods of the two rhythms. The authors examined the range of period differences over which unintentional visual coordination might occur in 16 participants (Experiment 1) and 15 participants (Experiment 2). Cross-spectral coherence analysis and the distribution of continuous relative phase revealed that visual entrainment decreased as the difference between participants' preferred period and the experimenter-determined period of the environmental stimulus increased. The present findings extend the dynamical systems perspective on person-environment coupling and highlight the significance of period difference to the emergence of unintentional coordination.