Our biodiversity has long been preserved, but the main constituents of our environment have been particularly affected by the addition of molecules resulting from agricultural and industrial activities. It is well accepted that these changes may stress some species, making them more vulnerable. In this project, we determined the disruptive side-effects of a pesticide on several biochemical endpoints and the behaviour of a microorganism as the ciliate protist Paramecium sp. Here we used fenazaquin [4-(4-tert-butylphenethoxy)quinazoline] that belongs to the quinazoline class of chemicals and that is a pesticide intended to control mites and insects; its route of exposure is ingestion and dermal, and its mode of action is the disruption of the biochemistry of insect mitochondria. In our experiments with fenazaquin at 40, 60 and 80 nM, we recorded disturbances in protein and glutathione, in glutathione S-transferase, and a decrease in consumption of oxygen. The results are discussed in relation to potential risks and mechanisms of action. In addition, the data can be used as reference values in further testing with other pesticides and chemistries.