The effects of darkness on startle reactivity and prepulse inhibition were investigated in two studies with 25 subjects participating in each study. Acoustic startle stimuli that were or were not preceded by an acoustic prepulse were delivered in alternating periods of complete darkness or light. In both studies, darkness significantly increased the magnitude of startle but did not affect prepulse inhibition (PPI). The PPI results suggest that darkness did not increase attention to the auditory modality, so that the startle facilitation in the dark probably did not result from an attentional process. The increased startle in the dark was significantly correlated with the intensity of subjects' fear of the dark as children based on retrospective rating scales. It is hypothesized that the startle facilitation in the dark results from a change in affect rather than from a change in attention.