Phenotypic plasticity, shaping responses according to environmental factors, is central to understanding the relationship between organism and their environment. Recent studies have shown that vertebrate growth can be plastic in response to biotic interactions such as competition and predation. It is unknown, however, if vertebrate growth plasticity occurs in response to mutualistic interactions. A good system to test this is the mutualism between anemonefish and anemone, since their sizes are often closely correlated. Here, we use a series of laboratory experiments and data from a wild population to test if the growth of clownfish, Amphiprion percula, is plastic in response to the size of their mutualistic partner. First, in the lab, we measured the growth of juvenile A. percula paired with surrogate anemone hosts, Entacmea quadricolor, of a range of sizes over 6 months. We found that fish in larger anemones grew faster than fish in smaller anemones, even though they received the same food rations. Next, we will repeat the experiment using fake anemones to test if the growth response is due to habitat area alone or if other components of the mutualism trigger the response. We predict that we will find the same growth response, indicating that A. percula is responding to the territory size provided by the anemone, which predicts foraging area in the wild. Finally, we took the predicted values from a mixed model fitted to the lab data, including initial size of fish and anemone size as predictors, and compared them to size data in a wild population of A. percula associated with Heteractis magnifica. We found high overlap in predicted and actual sizes. Together, we provide the first example of vertebrate growth plasticity in response to a mutualistic interaction, as well as possible fitness consequences.