Positive phototropism of perithecial beaks in the fungus Neurospora crassa has been demonstrated. The effect was shown to be mediated by blue light. When mutants (white collar-1 and white collar-2) which are blocked in the light induction of enzymes in the carotenoid biosynthetic pathway were used as the protoperithecial parent in crosses, the resulting perithecial beaks did not show a phototropic response. However, when wild type, albino-1, albino-2, or albino-3 strains were used as the protoperithecial parent, phototropism occurred.The results show that both photoinduced carotenogenesis and phototropism in N. crassa are controlled by the white collar-1 and white collar-2 loci. Thus, the sensory transduction pathways for the two photoresponses must have some steps in common. The results further support the proposal that the white collar strains are regulatory mutants blocked in the light induction process, whereas the albino-1, albino-2, and albino-3 strains can carry out light induction but have the albino phenotype because they are each defective for a different enzyme in the carotenoid biosynthetic pathway.