The objective of this study was to develop a rapid and effective method of sexing juvenile sea bass with minimum labour and material. To this end, the gonad squash mount technique was applied along with macroscopic techniques for sexing a large number of experimental fish at the age of 215-275 days post fertilization (p.f.). At this age, 90% of the 3894 fish could be unambiguously sexed by macroscopic examination of their gonads, whereas the remaining proportion was identified using squash mount preparations (10%). The accuracy of the observations was measured up to classical histology procedures. Undifferentiated fish accounted for 0.1% of the total population only. The application of the squash mount technique holds out the prospect of considerable improvement in the efficiency and rapidity of current sexing techniques in sea bass.