A $2^4$ full factorial design was used to identify the main effects and interactions of the initial medium pH, soybean flour concentration, temperature and orbital agitation speed on extracellular collagenase production by Penicillium aurantiogriseum URM4622. The most significant variables for collagenase production were soybean flour concentration and initial medium pH that had positive main effects, and temperature that had a negative one. Protein concentration in soybean flour revealed to be a significant factor for the production of a collagenase serine proteinase. The most favorable production conditions were found to be 0.75% soybean flour, pH 8.0, 200 rpm, and $28^{circ}C$, which led to a collagenase activity of 164 U. The enzyme showed an optimum activity at $37^{circ}C$ and pH 9.0, was stable over wide ranges of pH and temperature (6.0 ~ 10.0 and $25{sim}45^{circ}C$, respectively) and was strongly inhibited by 10 mM phenylmethylsulphonylfluoride. The first-order rate constants for collagenase inactivation in the crude extract, calculated from semi-log plots of the residual activity versus time, were used in Arrhenius and Eyring plots to estimate the main thermodynamic parameters of thermoinactivation ($E^*{_d}$ = 107.4 kJ/mol and ${Delta}H^*{_d}$ = 104.7 kJ/mol). The enzyme is probably an extracellular neutral serine collagenase effective on azocoll, gelatin and collagen decomposition.