The effects on catches and bycatches due to increases in mesh size were investigated in a bottom-set gillnet fishery for dusky flathead, Platycephalus fuscus in New South Wales, Australia. Sampling was done in 2 coastal lagoons during spring 2001 using a gillnet comprised of replicate panels of (i) the commercially-used mesh size (7.0 cm) and (ii) 3 larger mesh sizes (8.0, 8.9 and 9.5 cm). The targeted dusky flathead and 5 important bycatch species (bream, Acanthopagrus australis, luderick, Girella tri-cuspidata, sea mullet, Mugil cephalus, yellow-finned leatherjacket, Meuschenia trachylepis and blue swimmer crab, Portunus pelagicus) accounted for approx. 91 % of the total catch. Significant differences in the catch rates of these species were detected among the mesh sizes examined with trends for fewer numbers of total dusky flathead, total bycatch, sea mullet, yellow-finned leatherjacket and under-size bream and luderick evident in the larger mesh sizes. All species were characterised by a significant disequilibrium in the vertical distribution of catches, with the lower 50 % of the gillnet retaining between 56 and 89 % of individuals. Modal lengths of most species increased with mesh size and the selectivities of dusky flathead, bream and luderick were best described by binormal curves with well-separated modes. The results are discussed in terms of the probable capture processes of the main species and appropriate modifications to reduce their unwanted bycatches.