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Length frequency data of the longtail tuna, Thunnus tonggol collected from October 2006 to September 2007 were analyzed for evaluation of the stock parameters. T. tonggol is mainly caught by gillnet method. Yearly tuna and tuna-like catches in Iran are of the order of 174,234 mt, close to 46% of which are longtail tuna. The VBG parameters for longtail tuna were: L∞ =133.72cm K=0.35 year−1.The length –weight relationship was estimated as: W=0.00002 L 2.83.The instantaneous rates of total mortality (Z) was estimated by using the Powell-Wetherall plot as 1.82 year−1. The instantaneous rate of natural mortality was obtained by Pauly equation (M=0.44 year−1) and the instantaneous rate of fishing mortality (F) estimated from Z-M=1.38year−1.