The main aim of our research was to show that the body size of the planktonic cladoceran Daphnia cucullata can be used as an indicator of the ecological status of lakes. We selected this species because it is one of the most common zooplankton species in Europe and it exhibits a high degree of morphological variability depending on the environmental conditions. Additionally, D. cucullata is almost entirely ignored as a food source by fish unlike other Daphnia species, and often dominates in lakes where fish predation is intense. Therefore, we could minimize the top-down effects (fish pressure) and enhances bottom-up effects (food quality and availability) for this species. We measured the body length, body width, length of head, and length of tail spine of D. cucullata from 104 lakes with different trophic conditions in northern Poland. The results of our study revealed that D. cucullata grows larger in a better resource environment. The body length and width significantly decreased with increasing trophic status. The larger body classes of D. cucullata were absent in highly eutrophic conditions. Therefore, the body size of D. cucullata seems to be a good and simple indicator of the ecological status of lakes. We propose a simple formula for D. cucullata (Body Size Index) based on which we can assess the ecological status of lakes. We believe that our index could be applied to many other planktonic cladocerans.