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Knowledge of target strength (TS) is essential for accurate assessment of fish abundance using acoustic methods. The in situ TS (dB) of purpleback flying squid Sthenoteuthis oualaniensis in the South China Sea was measured with 70 and 120 kHz split-beam echosounders. Biological samples were collected using a light falling-net with a valid working depth of 50 m. Acoustic data sets in which S. oualaniensis contributed more than 90% of the total catch by number were selected to analyse in situ TS and determine relationships between TS and squid mantle length (ML). In situ TS increased with squid density when density was greater than a cut-off point, indicating a bias from accepting multiple targets at high squid density. At 70 kHz the TS to ML of squid was: TS 70 kHz = 19.13 log ML − 77.84 ( n = 9, R 2 = 0.8782, ML: 7.5–25 cm); at 120 kHz the TS to ML of squid was: TS 120 kHz = 26.68 log ML − 88.00 ( n = 9, R 2 = 0.8521, ML: 7.5–25 cm). This study provides new data for improving the acoustic abundance estimation of squid S. oualaniensis , and may be a useful reference for TS estimation of other squid species. [ABSTRACT FROM AUTHOR]