Effects of different concentrations of Diazinon on Roach Torkemani (Rutilus rutilus caspius) fries supplied from Ghareh-Soo Fishery Research Station was studied in Gorgan University of Agricultural Science and Natural Resources in 2009 to determine 50% lethal concentration (LC50) for the fish within 96 hours. In this study, 126 Roach fries with mean weight of 3 grams were used. Tests were performed statically based on instructions of O.E.C.D under fixed water quality conditions at the temperature 24±1 and pH 8-8.5 in a completely random trial with five concentrations treatments of Diazinon (5, 5.95, 7.07, 8.48 and 10mg/l) in three replications and the means were compared using Duncan test at 5% significance level. The results indicated that at the end of 96-hour test, LC50 was 7.88mg/l. In this test, clinical signs such as hunched spinal column, thrilling, clot in caudal fin and head, and irregular swimming were observed in the studied fish specimens. In hematological examination, there was a remarkable decrease in percent of hematocrit between concentrations of 5 and 5.95mg/l of Diazinon. By increasing the concentration of this poison, the decrease became slower.Furthermore, there was a significant difference in percent of hematocrit between control group and LC50 group, with mean amounts of 36 and 27%, respectively. Based on the results, Diazinon was classified as toxic pesticide for Roach Torkemani.