Aquatic animals, mainly those living near farms, factories, or cities, are subject to pollution caused by pesticides and/or chemicals. Since most pesticides are liposoluble and of widespread use, they affect adipose tissue, membranes and even lipoproteins. Fenitrothion is one of the most used organophosphorates; it is hydrophobic and affects many organisms that are not the target of action. Crustaceans are encountered within these aforementioned organisms, which though not very studied are considered of great importance for their great diversity in the animal kingdom. Within this group, the main representatives of the South American freshwater environment are Macrobrachium borellii and Palaemonetes argentinus. One of the aims of the present chapter is to report the scarce data related to the basal activities of biomarker enzymes, which can be used as tools for ecotoxicological studies and/or monitoring. Another objective is to give a clear example of the biomarking function of different enzymes, when two crustaceans are exposed to an organophosphorate. Cholinesterase activity is an example of a bioindicator of organophosphorates. Superoxide dismutase, catalase and glutathione S-transferases are examples of bioindicators because different pollutants can directly or indirectly modify the balance of the concentration of pro-oxidants and antioxidants.