Zinc oxide nanoparticles (ZnO NPs) have been widely employed in technological applications due to their appealing properties for catalytic, optoelectronic and antimicrobial applications. Moreover, ZnO NPs can also be employed in fertilizers as a source of zinc micronutrient, which is a primary element for enzymes and proteins synthesis in plants, allowing their regular growth and improving crop productivity. However, the extensive use of nanoparticles as a source of fertilizers in agricultural activities also raises concerns once nanoparticle features including surface charge, chemical fucntionalittes, size and shape can alter the toxicity of NPs. In this scenario, the toxicity of distinct ZnO NPs towards microalgae Pseudokirchneriella subcapitata, which is the base microorganism for the aquatic food chain, was evaluated and compared. ZnO NPs employed here were obtained by co-precipitation method and co-precipitation followed by hydrothermal method using different times. After characterizing the physical-chemical properties of ZnO NPs, they were employed in toxicity biossays in order to evaluate their ability to inhibit algal growth as well as to induce changes in algae morphology. Our results indicate that ZnO NPs concentrations below 10 mg L-1 presented low toxicity towards P. subcapitata microalgae. Higher concentrations of ZnO NPs, however, presented significant toxic effects, and revealed changes in morphology of the algae cells, and therefore, should be avoided for agri-related applications.