By screening for the osmotically remediable phenotype, mutations in two genes (orlA and orlB) affecting the cell wall chitin content of Aspergillus nidulans were identified. Strains carrying temperature-sensitive alleles of these genes produce conidia which swell excessively and lyse when germinated at restrictive temperatures. Growth under these conditions is remedied by osmotic stabilizers and by N-acetylglucosamine (GlcNAc). Remediation by GlcNAc suggests that the mutations affect early steps in the synthesis of chitin. Temperature and medium shift experiments indicate that the phenotype is the result of decreased synthesis rather than increased chitin degradation and that osmotic stabilizers act to stabilize a defective wall rather than to stabilize the gene product. Two genes, orlC and orlD, which affect cell wall beta-1,3-glucan content were also identified. Walls from strains carrying mutations in these genes exhibit normal amounts of alpha-1,3-glucan and chitin but reduced amounts of beta-1,3-glucan. As for the chitin-deficient mutants, orlC and orlD mutants spontaneously lyse on conventional media but are remedied by osmotic stabilizers. These results indicate that both chitin and beta-1,3-glucan are likely to contribute to the structural rigidity of the cell wall.