The pollution of aquatic bodies by pharmaceutical compounds is an emerging environmental problem, with little explored consequences. Oxytetracycline (OTC) is an antibiotic used for treatment of infections caused by a variety of microorganisms and it is widely employed in medicine, livestock husbandry and aquaculture. This pharmaceutical compound may cause deleterious effects on non-target aquatic organisms as microalgae. The objective of this study was to evaluate the effects of OTC on growth, pigment content and morpho-physiology of the microalga Isochrysis galbana Parke. The results highlighted that OTC exposure inhibited the growth of I. galbana in cultures treated with OTC 5.0 and 10.0 mg/L after 3 days and in cultures treated with OTC 5.0, 7.5 and 10.0 mg/L after 5 days. Effects of OTC on cells ultrastructure and physiology consisted in large cytoplasmic lipid inclusions and in a decrease of photosynthetic pigments amount. Copyright © 2020 Elsevier Ltd. All rights reserved.