AbstractThe response of the visual cell of the horseshoe crab Limulus to light stimuli is described. Due to the absorption of a single photon, a rhodopsin molecule is light‐activated and starts a sequence of causal steps which lead to the generation of a relatively large elementary excitatory response, the “bump”. A bump is based on the concerted transient opening of a large number of sodium ion‐preferring channels through the cell membrane. The bump generating mechanism is discussed. – The “macroscopic” light response to higher stimulus intensities is believed to consist of a fused volley of bumps. Its intensity dependence is discussed. A conditioning pre‐illumination usually desensitizes the visual cell transiently. However, very weak pre‐illumination may cause an increase in sensitivity, “facilitation”.