This paper describes results of a simultaneous inversion of bio-acoustic parameters of fish (anchovies) and geo-acoustic parameters of the bottom from transmission loss (TL) measurements in the Yellow Sea, which were reported by Qiu et al. [J. Sound Vib. 220, 331–342 (1999)]. This data set was selected because the bio-absorptivity at their site was extremely large, 40 dB at 1.3 kHz at 5 km, and measurements were made between multiple source and receiver depths and ranges. Measurements were made at night when anchovies are generally dispersed. Replica fields were calculated with a normal mode model, which incorporates bio-absorption layers. The inversion was based on minimizing the rms difference, Δ, between measured and calculated values of TL at all ranges and source and receiver depths, and involved a simultaneous search for bio-layer depth, bio-layer thickness, bio-alpha, geo-sound speed, and geo-alpha. The resultant small value of Δ, ±1.7 dB, confirmed that the model, which was assumed in replica field calculations, was realistic, and that inverted parameters were meaningful. In particular, the inverted depth of the bio-absorption layer, 6.9±0.3 m, was consistent with theoretical calculations of the depth, 5.8±1 m, of 10-cm-long anchovies Engraulis japonicus, the dominant species in the Yellow Sea. © 2005 Acoustical Society of America.