A series of 6 xanthene dyes—rose bengal, phloxin B, erythrosin B, eosin yellowish, fluorescein, and rhodamine B—caused a light-dependent toxicity in the house fly,Musca domestica, after oral ingestion. Relative toxicities were described by means of rate constants of photooxidation calculated for each dye which include both the LT50and the tissue dye level. These values were compared with similar values determined for reactions in the boll weevil,Anthonomus grandis, and with the phosphorescence quantum yields of the dyes, the number of halogen substituents, and the halogen content by weight. Lowered mortality at higher dietary concentrations was due to lowered consumption and was taken as preliminary evidence of feeding inhibition.