Many animals form dynamic societies in which the fission and fusion of subgroups occurs on a regular basis. Such societies are intriguing, because it is unclear whether stable dominance relationships that form within subgroups are retained upon fusion with other subgroups, and what the implications of rank instability may be at higher levels of ecological organization. Additionally, little is known about how environmental change affects the fission¿fusion process, even though environments often fluctuate and are predicted to become increasingly variable, in part due to climate change. Here we investigated the social organization, levels of conflict and stability of dominance relationships during group fusion in a globally invasive fish, the eastern mosquitofish, Gambusia holbrooki. To assess the effect of environmental variation, we conducted group fusion experiments at high and low water depths, to simulate normal and drought conditions, and recorded dominance interactions during prefusion, early fusion and late fusion stages. Individuals formed size-based hierarchies within prefusion groups, although the relationship between size and dominance varied with group fusion. Levels of conflict were affected by group fusion and water depth, with higher levels of conflict after fusion and at low depth. Rank relationships in early and late fusion groups were stable and unaffected by water depth. Finally, there was no evidence of coat-tail effects, as familiarity with the alpha dominant in postfusion groups did not lead to a significant increase in subordinate dominance rank. All in all, these results provide key experimental evidence that environmental change in terms of water level is unlikely to impact social organization or rank stability in response to group fusion in this species. More generally, they indicate that sociality in fission¿fusion societies may be relatively robust to changes in both social and environmental contexts.