Extracts of nitrate-grown mats of Aspergillus nigerNRRL3 catalyzed, with comparable efficiency, hydrolytic deamidation of both L-glutamine and L-asparagine in a similar manner under different experimental conditions. Other amides such as acetamide, nicotinamide and nicotinamide adenine dinucleotide could not be deamidated by these extracts. Optimum deamidation of the two amino acids by the extracts was recorded at pH 6 of 0.08 to 0.16 M phosphate buffer and 60°C. Exposure of the extracts to 60°C for 20 min in absence of the substrate revealed about 70% increase in each of the two activities. Fractionation of the extract proteins using DEAE Sephadex A-25 column chromatography showed that the glutaminase and asparaginase activities were detected in the same protein fractions with the same peak and with almost constant ratios. The ratio of the glutaminase activity to that of the asparaginase was approximately 0.7 to 1.0. The activities of the pooled fractions towards L-glutamine and L-asparagine showed similar behaviors at different pH values and at different temperatures. Both activities exhibited hyperbolic substrate saturation kinetics and the apparent km values were found to be 5.05 and 2.57x10-3 M for L-glutamine and L-asparagine, respectively. The sum of the two activities was much less than additive on using an equimolar mixture of L-glutamine and L-asparagine as substrates. Each of the two activities was not inhibited by the products of the reaction but inhibited by Mg++ or Hg++. Both activities were inactivated at almost the same rate on exposure to temperatures above 40°C and were retained in the freezer for some months. The same type of enzyme was also suggested to occur in a strain of Penicillium chrysogenum and in a strain of Penicillium politans.