This study evaluated the sensitivity of male and female of guppies (Poecilia reticulata) to ammonia. LC50-96h for males, females and combined sexes of P. reticulata was 37.33, 48.34, and 42.45 mg/L of total ammonia or 1.03, 1.34, and 1.17 mg/L of un-ionized ammonia. The mortality at the concentration of 40 mg/L of total ammonia differed between the sexes with higher mortality in males. These results have implications for production systems, since recommendations of ammonia toxicity reference values that do not consider the difference between the sexes, can lead to the mortality of males, and cause economic losses because males have higher commercial value in the ornamental market.