The present experiment investigated how deindividuation affects group polarization in computer-mediated communication. Before exchanging their opinions about social dilemmas with 3 ostensible partners via computer, participants either shared some personal information (individuated) or not (deindividuated). Consistent with the social identity model of deindividuation effects (SIDE), deindividuation fostered group identification with the partners and induced greater opinion polarization, partly by heightening concerns about public evaluations. Although participants rated the partners’ arguments more positively when they identified with the partners, perceived argument quality did not significantly affect postdiscussion opinion shift. Deindividuation did not lower private-self-awareness, nor did private-self-awareness significantly influence opinion polarization. Implications are discussed in light of SIDE and the referent informational influence theory.