Ectoine (ECT) is an osmoprotectant produced and accumulated by halophilic bateria under hyperosmotic stress. There is a lack of knowledge on the effect induced by this amino acid during subchronic animal exposure to ECT in vivo. Therefore, the aim the present study was to determine influence of ECT on survival, motility, physiological activity (heart rate, thoracic limb activity, and mandible movement rate), growth rate and moulting of unfed and fed with normal diet Daphnia magna exposed for 10 days to 2.5, 4, 20 and 25 mg/L ECT. The results showed that survival of unfed animals exposed to 25 mgL ECT was decreased; however it was increased at lower concentrations of the amino acid when compared to the control. 20 and 25 mg/L of ECT significantly decreased swimming velocity, heart rate and thoracic limb activity and mandible movement rate. On the other hand, the osmoprotectant did’nt cause mortality of fed daphnids at any concentration but stimulated swimming velocity and the physiological parameters, however no differences were found in mandible movement rate. We found that ECT increased growth rate both in starving and fed daphnids, however the stimulatory effect was not concentation-dependent. The most significant increase of growth rate both in unfed and fed dapnids was found at 4 mg/L ECT. Moulting frequency was increases only in unfed daphnids exposed to the highest concentrations of ECT. Fed daphnids showed slighlty elevated moulting frequency only at 25 mg/L ECT. Our results indicate ECT is a stimulator of growth rate and physiological indices in fed Daphnia magna, however the elucidation of its mechanisms requires further studies.