A laboratory bioassay was developed to determine both the chemical toxicity and the phototoxicity of the xanthene dye, phloxine B (D&C Red N°28), to the immature stages of the Mediterranean fruit fly, Ceratitis capitata (Wiedemann). An additional goal was to find out which main tissues are affected first. A low, but significant, level of toxicity was observed when the insects were maintained in the dark: at the point of adult ecdysis, the LC₅₀ was 11.03 mM. As expected, after 8-h exposure of late larva III to light, a high level of mortality was produced (LC₅₀ at ecdysis: 0.45 mM) as a dose-dependent function of dye concentration. At sublethal concentrations of the dye, the surviving insects showed a number of physiological abnormalities. Phloxine B appeared to mainly affect the larval longitudinal muscles as well as the abdominal muscles of ecdysing adults, giving rise to abnormal puparia and failed adult ecdysis, respectively. Moreover, a significant phloxine B-dependent delay in the jumping of surviving larvae for dispersal was documented. This could be attributed to a delay in attaining a threshold weight for jumping and/or to abnormalities in neuromuscular coordination, thus reinforcing the idea of pleiotropic effects of the dye.