The technique described here relates to the fixation of fish in formaldehyde under hyperbaric conditions. Samples are placed in a chamber containing 5% formaldehyde. A film of oil is provided to create an interface between the liquid and the 2.5 bar air atmosphere. The minimum contact time at this pressure is 24 h. The tissues are then desaturated in stages. At the end of the cycle the samples are rinsed and placed under ethanol. The formaldehyde is neutralized. All of the operations are performed with the principal concern being to expose the researcher to as little formaldehyde vapor as possible.