Cultivation of cyanobacterium Arthrospira sp. has been developed in many countries for the production of proteins, pigments and other compounds. Outdoor mass cultures are often affected by biological contamination, drastically reducing productivity as far as bringing death. This study evaluates the control of Branchionus sp. and Amoeba sp. with two chemical compounds: urea (U) and ammonium bicarbonate (AB), in laboratory conditions and outdoor mass culture of Arthrospira sp. The lethal concentration 100 (LC100) at 24 h for Branchionus sp. and Amoeba sp. determined was of 60-80 mg L-1 (U) and 100-150 mg L-1 (AB). The average effective inhibition concentration for 50% of the population (IC50) in Arthrospira sp., after 72 h, was 80 mg L-1 (U) and 150 mg L-1 (AB). The application of doses of 60 mg L-1 (U) or 100 mg L-1 (AB) in the outdoor mass culture of this contaminated microalga, completely inhibited grazing and did not affect the growth of Arthrospira sp. but rather promoted rapid recovery of algal density at levels prior to infestation. These compounds provided an economical and effective control of predators in cultures of Arthrospira sp.