The effect of moisture on reduction of the flow starting temperature (T-f; i.e. start of flow through a capillary die at constant pressure conditions) in major protein ingredients used in fish feed has been assessed using a Phase Transition Analyzer. All proteins showed an initial linear reduction in T-f followed by a sharp sigmoidal reduction at higher moisture content. A critical moisture level (M-cr) could be defined at the onset of this transition region. Fishmeal and wheat gluten show the lowest M-cr (138 and 160 g kg(-1), respectively) and highest reduction in T-f above M-cr. Sunflower meal and soy protein concentrate demand more moisture, show higher variability in M-cr (210-262 and 233-306 g kg(-1), respectively) and less reduction in T f at moisture levels above M-cr. The T-f level corresponds to an apparent viscosity of 10(5) Pa s and can be combined with the William-Landel-Ferry equation to establish moisture contour plots for the effect of temperature on viscosity reduction in the rubbery state. The protein specific M-cr levels and rheological profiles can be applied to determine optimal moisture and temperature conditions for plasticization and texturization of extruded food and feed products.