Group-living animals must synchronize their behavior to maintain group cohesion. Synchrony is best maintained by individuals similar in size because they have similar activity budgets and thus can easily coordinate their behavior, which minimizes potential trade-offs. Many groups assort by size, presumably to reduce predation risk and foraging competition. However, because groups can only be maintained by individuals that synchronize their behavior, we propose that maintenance of synchrony may also be an important factor contributing to size-assorted grouping. Similar-sized individuals should maintain synchrony through similar swimming speeds, similar foraging activity and through cohesiveness with the group, and, therefore, pay the least costs of group-living: body mass should increase over time. However, odd-sized fishes may have to adjust their foraging activities and thus lose body mass because of the need of synchrony. We measured the aforementioned variables using varying sizes (large and small) and colors (red and wild-type) of heterogeneous (1 odd focal fish among 5 similar fishes) and homogeneous (6 similar fishes) groups of zebra fishes. Groups composed of similar-sized individuals had the highest degree of synchrony--small fishes gained mass, whereas large fishes grew very little or not at all. Groups composed of different-sized individuals, although able to maintain synchrony most of the time, did so at significantly lower levels than similar-sized fishes--odd small fishes in these groups gained significantly less mass, whereas odd large fishes did not. We show that synchrony in behavior is costly and that these costs may contribute, in part, to group choice. Copyright 2011, Oxford University Press.