A cosmid carrying the orIA gene from Aspergillus nidulans was identified by complementation of an orlA1 mutant strain with DNA from the pKBY2 cosmid library. An orlA1 complementing fragment from the cosmid was sequenced. orlA encodes a predicted polypeptide of 227 amino acids (26 360 Da) that is homologous to a 211-amino-acid domain from the polypeptide encoded by the Saccharomyces cerevisiae TPS2 gene and to almost the entire Escherichia coli of otsB-encoded polypeptide. TPS2 and otsB each specify a trehalose-6-phosphate phosphatase, an enzyme that is necessary for trehalose synthesis. orlA disruptants accumulate trehalose-6-phosphate and have reduced trehalose-6-phosphatate phosphatase levels, indicating that the gene encodes a tre-halose-6-phosphatate phosphatase. Disruptants have a nearly-wild-type morphology at 32°C. When germinated at 42°C, the conidia and hyphae from disruptants are chitin deficient, swell excessively, and lyse. The lysis is almost completely remedied by osmotic stabilizers and is partially remedied by N-acetylglucosamine (GlcNAc). The activity of glutamine:fructose-6-phosphate amido-transferase (GFAT), the first enzyme unique to aminosugar synthesis, is reduced and is labile in orIA disruption strains. The findings are consistent with the hypothesis that trehalose-6-phosphate reduces the temperature stability of GFAT and other enzymes of chitin metabolism at elevated temperatures. The results extend to filamentous organisms the observation that mutations in fungal trehalose synthesis are highly pleiotropic and affect aspects of carbohydrate metabolism that are not directly related to trehalose synthesis.