The steady-state extensional growth rate of five unknown (un, TS) mutants of Neurospora crassa at various temperatures was measured. Data from Dr. R. K. Littlewood on growth of two extrachromosomal mutants are presented for comparison. The optimum growth temperature of six of the seven mutants is 5-10° lower than that of wild-type. The free energies of inactivation of the mutants are significantly greater than that of wild type. Six of the seven also exhibit relatively large energies of activation. Thus, all of the mutants are "hot" sensitive and six of the seven are also cold sensitive. All of the TS mutants are osmotically remedial at either high or low temperatures or both. Biochemical data indicate that at least some nutritionally irreparable, temperature-extremity sensitive, osmotically remedial mutants may be defective in cell wall or membranes; however, there are reasons to believe that this phenotype is not unique to such lesions.