Biological cells sense and react to their environment. This is as true for a neuron in the brain as for a bacterium in the intestine or a coprophilous Zygomycete on dung. To understand these cells on a molecular level scientists have choosen particu­ lar systems for extensive study. Such a system must respond to measurable stimuli, give clear responses, and be convenient for laboratory use. For this review, light is the easily controlled stimulus and the bending of plant cells to light, phototropism, is the observed response. The cells bend because locally on one side of the cell there is greater growth or growth rate than on the other side. I have limited this review mostly to fungi because, unlike plants that photosynthesize, light acts in them only as a sensory stimulus and not as its source of energy as well. I have chosen structures that are single cells because mediation of the output response via the interaction of cells creates another layer of stimulus-response interaction. Multicel­ lular systems do have the advantage of membranes between parts, allowing, for example, determination of the movement of growth hormones for study of the biochemistry of cell wall growth. It is also necessary that the cells grow well under laboratory conditions. These considerations leave us with the coprophilous fungi of the class Zygomycetes of which the best known and most studied by sensory physiologists is Phycomyces (1). The spore-firing Pi/obolus (which shoots spores more than 2.5 m horizontally), Pilaira, and Conidiobolus will also be discussed, with a few brief diversions to non-Zygomycetes such as Vaucheria, Pteridium, Dryopteris, Fucus, and Botrytis to illustrate specific points. Even in a relatively uncomplicated cell such as Phycomyces, which has a DNA content only 6.7 times that of the bacterium Escherichia coli (2), a number of processes take place between the reception of light and modification of its growth rate. Simultaneously in parallel with this pathway occur a number of possible interactive processes. In addition to regulating growth rate, light receptors receive