Intra-and interpersonal inter]imb coordination of pendulums swung from the wrist was investigated. For both kinds of coordination, the steady state and breakdown of bimanual rhythmic coordination as indexed by the time series of the relative phase angle it were studied under the manipulation of coordination mode, frequency of oscillation, and the difference in the eigenfrequencies (preferred tempos) of the individual oscillating limbs. The properties observed for both intra-and interpersonal coordination were those predicted by a dynamical model of rhythmic coordination that considers the coordinated limbs coupled to be nonlinear oscillators. Using a regression method, the coupling strengths of the coupled system were recovered. As predicted by the dynamical model, the strength of the dynamic was generally greater for the in-phase than the anti-phase mode and decreased with increasing frequency. Further, the strength of the interpersonal interlimb coupling was weaker than that of intrapersonal interlimb coupling.Much research over the past decade has demonstrated that stable patterns of interlimb coordination and their dissolution can be modeled by assuming that the nervous system and concomitant metabolic machinery manifest dynamical processes of self-organization (Haken, Kelso, & Bunz, 1985;Kelso, 1990;Kugler, Kelso, & Turvey, 1980;Kugler & Turvey, 1987;Turvey, 1990). The methodological tactic of this research has been to demonstrate that the patterns of rhythmic coordination observed in single (Feldman, 1980;Kay, Saltzman, & Kelso, 1991) and multilimb (Beek, 1989;Kelso & Jeka, 1992;Schmidt, Shaw, & Turvey, 1993) movements are just those expected if the neuromuscular systems involved are governed by the dynamics of nonlinear oscillators. Interestingly, research has also demonstrated that patterns of coordination similar to those found in withinperson interlimb coordination occur in rhythmic movements optically coordinated across two people (Schmidt, Carello, & Turvey, 1990;Schmidt & Turvey, 1994). These latter findings demonstrate the generality and abstract incorporeal nature of the organizing principles involved in biological rhythmic coordination: The same control processes seem to be at work regardless of whether the coordination involves one nervous system or two or whether the coupling medium is the optic array or neural tissue. In two experiments we further compared the coordination dynamics found in intraand interpersonal interlimb rhythmic movements, particularly the strength of those dynamics and their patterns of coordination breakdown. We did this using a methodology that manipulated the frequency of oscillation and the dynamical similarity (i.e., the preferred frequency or eigenfrequency difference) of the rhythmic units being coordinated. We also present a new method of measuring the strength of the coupled oscillatory dynamic.The research on the dynamics underlying interlimb rhythmic coordination has used the relative phase angle (