In this article, we investigate the effects of avatars' appearance on user social behavior in online virtual worlds. In particular, we study appearance differences in social communication preferences and behavior in virtual public spaces. For this purpose, we have employed virtual ethnographic methods, which is an adaptation of traditional ethnography for the study of cyberspace. We employed nine users who used four different avatars and we observed a cumulative of more than two hundreds social encounters. The results of the study indicate that avatars' appearance is an important factor in determining the social communication patterns between users in online 3D worlds. In particular, we found that users with more elaborate avatars had a higher success rate in their social encounters, than those users with the default avatars. The implications of this study raise several issues for the design of avatars, as well as for the study of social communication in online 3D worlds. appearance, sociability.