Two temperature-sensitive genes are described which affect conidiation in Neurospora crassa. One (acont) decreases the length of aerial hyphae at 35[degree] but not at 25[degree]. Those strains (col-1; msum; acont and col-1; m+; acont) which have short aerial hyphae at 25[degree] are aconidiate at 35[degree]. Growth on a rich conidiating medium results in the non-expression of acont in some but not all of the acont strains tested. To explain the observations on conidiation in the acont strains it is suggested that acont is active during a certain period in the ontogeny of the culture, this period being independent of genotype and environment. Another genotype col-1; m; acont+ has a phenotype which is dependent on the incubation temperature. At 25[degree] it is microconidiate but at 35[degree] it is macroconidiate. Since col-1; m+ strains are aconidiate, the m gene determines whether any conidia shall be produced, but the type of conidia formed is determined by the ambient temperature. The sum gene removes this environmental control of conidial phenotype, the col-1; m sum strains being macroconidiate at both temperatures. The sum gene may be described therefore as a canalization gene.