Sporangiophores (spphs) of Phycomyces blakesleeanus are positively phototropic to unilateral visible (blue) light over a range of fluence rates 10$^{-9}$ to 1 W/m$^2$. The maximal bending angle is always 70-75$circ$ from the vertical. Many mutants with abnormal phototropism have been isolated. Complementation tests revealed that the genetic grouping is completely consistent with the phenotypic classification scheme, based on sensory responses other than those to light. The spph of the piloboloid mutant, the growth zone of which gradually ceases elongation but expands spherically, and the $eta$-carotene-overproducing mutant show negative phototropism, in contrast to the wild type spph. We hypothesized that the phototropic orientation of spph is determined by the ratio of the maximal light fluenee rate at the proximal side to that at the distal side of the spph. Based on this hypothesis, we found that the maximal bending angle was larger in thin spphs than in thick ones, and larger in spphs containing smaller amount of $eta$-carotene than in carotene-rich spphs. In addition to our hypothesis, gravitropic experiments revealed that the maximal bending angle of the wild type spph results from a balance among positive phototropism, negative gravitropism, and the optical properties of the spph. For further advancement of this study, we developed a mutant with a high proportion of uninucleate spores, and designed an efficient microinjection method for obtaining transformants.