Abstract—Studies were carried out on the toxicity of iron on the physico-chemical and biological parameters of oxidation ponds which are used as one of the biological treatment method to purify wastewater. The level of pH, DO and density of algae and protozoa were decreased and BOD and phosphate concentrations were increased as the concentration of iron was increased in the oxidation pond. The percentage removal of BOD on day 20 was 62.5%, 56.9%, 43.8%, 32.5% and 28.8% respectively in control, 25 mg/l, 50 mg/l, 75 mg/l and 100 mg/l of FeSO4 treatment. The percentage removal of phosphate was 40%, 37.5%, 35% 30% and 25% respectively in control, 25 mg/l, 50 mg/l, 75 mg/l and 100 mg/l of FeSO4 treatment. The activity of enzymes namely catalase, amylase, protease and phosphatase were reduced in ponds treated with higher concentration of iron. The assessment of iron toxicity on the predominance of algae and protozoa showed that Scenedesmus acuminatus and Vorticella campanula were recorded as most tolerant species of alga and protozoa respectively, whereas Oscillatoria brevis and Podophrya fixa were recorded as the most sensitive species of alga and protozoa respectively for ferrous sulphate treatment.