The light-activated toxic effect of erythrosin B on apple maggot fly was studied in the laboratory. Flies were allowed to feed in the dark on an aqueous solution of erythrosin B in 10% sucrose and were then exposed to light. Mortality, observed at the end of a 6-h exposure period, increased with dose of erythrosin B over the range 500–2,000 ppm but leveled off at higher doses. Mortality also increased with exposure time at 1,000 and 2,000 ppm. Data on survival versus exposure time were analyzed using a logarithmic model previously proposed, but apparently never used, to describe the process by which insects succumb to dye-sensitized photooxidation. With the addition of a parameter representing an exposure-time threshold, this model fit our observations on apple maggot better than the pro bit model, which is usually but we believe inappropriately, applied to data in studies of this type. Insects continued to die after they were returned to the dark following exposure to light, suggesting the existence of a residual effect of the light exposure. This calls into question the standard practice of assessing mortality immediately following the period of light exposure in studies of dye-sensitized photooxidation.