Interspecific aggression, similar to intergroup conspecific aggression, has been observed in a variety of taxa. The dominant group or individual is determined by multiple aggressive events and can be influenced by the size, age, or group size of the participating individuals. Interspecific aggression between Atlantic bottlenose (Tursiops truncatus) and spotted (Stenella frontalis) dolphins, both resident and sympatric to Little Bahama Bank, the Bahamas has been consistently observed for over two decades. However, it is unclear whether one species is more dominant and little is known about the factors that influence the progression of aggression. For this study, underwater video recordings of 32 aggressive encounters composed of 451 aggressive behavioural events were analysed over a 12-yr period (1993–2004). These were used to describe the interspecific aggression observed and quantify which factors (the species and age class of the participants or the group size and behaviour of spotted dolphin groups) had the strongest impact on the progression and outcome of aggression. Over the long term, interspecific aggression was bidirectional with neither species being more dominant. During a single encounter, spotted dolphin group synchrony had the strongest impact on the dynamic of aggression, specifically impacting which group (1) initiated aggression, (2) the direction of aggression and (3) the occurrence of dynamic shifts or dominance reversals. This is the first study to quantify the dynamic of aggression for this population, to document bidirectional aggression and dynamic shifts during long-term interspecific aggression in free-ranging delphinids, and this study quantifies the role of synchrony during interspecific aggression using underwater observations.