A transparent testing cell was constructed to assess flow of fish tissue during a compression-back extrusion mechanical assay. The net action of the plunger over the fiber-interfibrillar collagen matrix appears to involve flow of intact fibers due to yield of the matrix as well as compression resisted by fibers. The force at yield point F (N) coincided with initiation of flow and followed a power relationship with sample weight W (g) of the type F=2.76W1.21. Mechanical assays after incubation at 30°C showed that light/dark tissue had higher yield point values than light tissue, with a strong effect of time of incubation, temperature and pH. The proposed method was effective in assessing softening of the collagen matrix in fish tissue without destruction of the fibers.