Abstract Young red tilapias were exposed for 96 h to each one of 6 concentrations of the pesticide Diazinon in order to determine the pesticide's acute toxicity level. After the ascertaining the lethal concentration (LC 50 ) at 96 h, a level 10 times lower was selected for the bioaccumulation study of the pesticide in male and female specimens exposed for 9 days. The elimination process was carried out for 10 days beginning right after the conclusion of the accumulation process. Analytical procedures were developed and used for the studies of acute toxicity and bioaccumulation of Diazinon in red tilapia. A lethal concentration [LC 50 (96 h) ] of 3.85 mg/L was found, and steady‐state accumulation, at a concentration of 28.45 mg/kg, was reached at 7.72 days. In the elimination process a concentration of 0.29 mg/kg was found in tilapia tissue by the sixth day after the fish were moved to clean water, and it continued to decrease quickly toward nondetectable levels. © 2002 Wiley Periodicals, Inc. Environ Toxicol 17: 334–340, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/tox.10063