Infestation of gregarines from genus Nematopsis in Pacific white shrimp Litopenaeus vannamei is commonly associated with a decrease in production and low weight, as well as a possible predisposition to viral infections. The goal of our past research was to evaluate the effect of the Nematopsis sp. gregarine parasitosis on the growth of farm-grown Litopenaeus vannamei shrimp. Four cages were built with PVC pipes and plastic screens with a 6mm light diameter, 1,7m in height, and 1,5m in width and length (2,23 m3), and were installed in a farm-growing area in La Pesca, Tamaulipas. Twenty-nine previously weighed shrimp, L. vannamei, were placed in each cage. These had a “2” (6 to 10 parasites per shrimp) degree of infestation severity by Nematopsis sp. The shrimp in 2 cages were fed for 5 days with the farm feed and 6g/kg of sodium monensin. The shrimp in the remaining cages were fed with commercial feed. After 5 days, the shrimp in the 4 cages were fed with a balanced feed for 38 days. After this, the biomass of each cage was recorded. The shrimp in the control cages weighed an average of 7,12g at the beginning of research and 9,27g at the end, and the treated shrimp weighed 7,09g and 9,88g, respectively. The non-treated shrimp gained 2,5g and the treated shrimp gained 2,79g. This present research shows that the negative effect on the shrimp growth caused by Nematopsis sp. infestation can be reversed if the infected shrimp are treated against infestation with sodium monensin, which possesses a therapeutic effect that eliminated the infestation without affecting the shrimp growth.