The social microcosm is defined as group members replicating their everyday (intersession) interpersonal behaviors in group sessions and new behaviors, learned in the group (in-session), replicating in the members' everyday life. We examined intersession and in-session intimate behaviors, at the within-member (differences in intimate behaviors between weeks/sessions), between-member (average differences in intimate behaviors between group members) and between-groups (group-level differences in intimate behaviors). Participants were 178 graduate students (86% identifying as women and 14% as men) participating in 10 5-session growth groups led by experienced group therapists. Before group sessions, group members completed the Interpersonal Relations Scale Checklist (IRScl; Shadish, 1984) indicating their number of intersession intimate behaviors for the previous week and, at the end of group sessions, they filled in the IRScl to indicate their in-session intimate behaviors. A 3-level HLM analysis (sessions, members, groups) predicting in-session intimate behaviors from previous week intersession intimate behaviors showed significant within-member, between-member, and between-groups effects. A second 3-level HLM analysis (sessions, members, groups), predicting following week intersession intimate behaviors from in-session intimate behaviors, showed significant between-member and between-groups effects. Between-member and within-member in-session intimate behaviors interacted to predict intersession intimate behaviors. Group members who generally had a low number of in-session intimate behaviors engaged in more intersession intimate behaviors in weeks following sessions with higher than average in-session intimate behaviors. These results provide support for the social microcosm proposition that members' trait-like everyday behaviors are replayed in the group. However state-like and other-member everyday behaviors also contribute to members' social microcosm. (PsycInfo Database Record (c) 2021 APA, all rights reserved).