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The Western flower thrips Frankliniella occidentalis, an important greenhouse pest, has acquired rapid resistance to the chemical pesticides. Therefore, biological control is worth consideration as an alternative control method. Among the biological control agents, entomopathogenic fungi showed to be quite successful in some occasions. In this study, three Iranian isolates of Metarhizium anisopliae (‘DEMI001’, ‘DEMI002’ and ‘DEMI003’) were bioassayed for their lethal effects on the adults of the F. occidentalis, in vitro. The ‘DEMI002’ and ‘DEMI003’ had the lowest and highest LC₅₀ at concentrations of 3.06 ´ 10⁴ and 1.90 ´ 10⁵ conidia/ml, respectively. Also, the isolate ‘DEMI002’ had the lowest LT₅₀ of 4.39 ± 2.13 days at the concentration of 10⁶ conidia/ml. The mean comparison showed that there was a significant difference between DEMI002 and DEMI003 in terms of virulence at most of the concentrations. Consequently, the ‘DEMI002’ can be considered as a promising tool in biological control programs of the F. occidentalis.