The susceptibility of Eretmapodites quinquevittatus eggs and larvae to photosensitization with Photofrin II (PII) was determined. Hatching rates of eggs (5 min to 2 h after oviposition with no PII exposure) exposed to solutions of PII (90 micrograms/ml) in the dark for 12 h, before photoirradiation for 72 h. were very similar to those of untreated controls (P > 0.05). Hatching rates of eggs (5 min after oviposition with no PII exposure) exposed to PII (90 micrograms/ml) and photoirradiated simultaneously for 72 h were significantly lower than for untreated controls (P < 0.05). Permeability of the egg to PII diminishes as the eggs age and the serosal cuticle becomes mature. Eretmapodites quinquevittatus larvae were killed after exposure to PII (90 micrograms/ml) that had been previously photoirradiated for 1-11 days. The effect was age and size related. First-instar to early 3rd-instar larvae were killed when exposed to PII (90 micrograms/ml) with no photoirradiation.