Production of the extracellular milk-clotting caseinase enzymes by 7 fungal cultures was investigated. Production of these enzymes was better in static than in shaken cultures. The sequence of production of these enzymic activities during incubation differed according to the age and type of the culture. There was considerable Variation in the ratio between the milk-clotting activity and proteolytic activity from one organism to another and, for the same organism, with the age of the culture. Penicillium oxalicum in 8-days-old static cultures was the most promising fungus as a rennin producer. The highest yield of milk-clotting enzyme was achieved by using sucrose (5%) as a carbon source and a mixture of yeast extract and peptone, or baker＇s yeast, as nitrogen source. Substrate concentrations seemed to affect the rate of the enzymic stage of rennin action and hence clotting of milk. The enzyme action was optimal at 60 degrees C and at about pH 6.0. In absence of substrate and up to 40 degrees C the enzyme was highly stable, at least for 30 min, retaining 75% of its original activity.