Ectoine (ECT) is a compatible solute synthesized mostly by halophilic microorganisms subjected to various stressful factors. Its protective properties in bacteria and some populations of isolated cells subjected to different stressors are reported; however, little is known on its effects against a commonly used compound, ethanol (ETH). The purpose of our study was to determine the effects of ETH alone (at 20 and 60 g/L) and in the combination with various concentrations of ECT (5, 10, and 25 mg/L) at various times of exposure on behavioural, physiological, and biochemical parameters of a model invertebrate Daphnia magna. In the present study, we determined the following parameters: immobilisation, heart rate, thoracic limb movement, catalase (CAT) activity, and nitric oxide species (NOx) level. Our study revealed that both concentrations of ETH alone induced immobilisation and decrease of swimming velocity, heart rate, and thoracic limb activity; however, catalase activity and NOx levels were increased. On the other hand, the animals exposed to the combinations of ETH + ECT showed a reduced immobilisation and alleviated inhibition of heart rate and thoracic limb activity, lower increase of CAT activity, and NOx level when compared to the crustaceans subjected to ETH alone. The most distinct alleviation of toxic effects was noted in the combinations in which the highest concentration of ECT were used. The results suggest that ETH may induce oxidative stress in daphnids and attenuating effects of ECT probably result from its antioxidative properties.