Stoichiometric homeostasis refers to the ability of organisms to keep their somatic elemental composition constant in the face of resource variation. Homeostatic strength has important implications for nutrient cycling and trophic transfer efficiency. Homeostatic strength varies considerably among and within species but its adaptive costs and benefits have received little attention. While strict homeostasis may be expected to secure optimal body functioning in the face of stoichiometric mismatch, it comes with the disadvantage of growth being constrained by the element in shortest supply and the need to efficiently discard excess elements. Conversely, plasticity, the opposite of homeostasis, reflects flexibility regarding the utilization efficiency of the limiting element and storage capacity of excess elements but may also reflect an inability to deal with elemental imbalance, especially when related with fitness reductions. To evaluate the adaptive benefit associated with homeostasis, we cultured 14 clones of the planktonic herbivore Brachionus calyciflorus with phosphorus sufficient (HP) and deficient algae (LP) and measured somatic P content and population growth rate. Genotypes with the highest somatic P in HP had highest fitness but suffered the strongest reductions in somatic P and fitness in LP. Yet, these genotypes remained to be those with highest P content and fitness in LP. Therefore, although homeostatic strength is associated with smaller fitness variation, constitutive P content is a more important determinant of relative fitness than homeostatic strength.