We localized one or more aldehyde dehydrogenase (ALDH)-like proteins in the bacterially bioluminescent light organ of the sepiolid squid Euprymna scolopes, and determined the temporal changes in expression through normal light organ development. Our previous studies have revealed that 70% of the total protein in the light organ lens of adult animals is comprised of an ALDH-like protein, which we called L-crystallin. In the present study, antibodies raised to this protein were used in immunocytochemical analyses which showed that, in adult light organ lens cells, ALDH-like protein was localized to the cytoplasm, but not to the nuclei or mitochondria. Labeling in adult tissue was also found in moderate abundance in the ciliated duct epithelium, a tissue that is in direct contact with the bacterial symbionts. To determine the spatial and temporal onset of expression of ALDH-like protein(s), we examined light organs from juveniles at developmental stages before and after the differentiation of lens cells, which begins approximately 7-10 days after hatching. In 5-day symbiotic juvenile light organs, ALDH-like protein was not detected at levels significantly above those in non-symbiotic tissue of the same animals. However, expression of ALDH-like protein began within 10 days after hatching, seen first in a few cells of the ciliated duct, adjacent to the symbiont-containing tissue and in a few differentiated cells of the anterior presumptive light organ lens. These data suggest that, during normal development, induction of one or more ALDH-like proteins occurs simultaneously in both the lens and ciliated duct soon after the differentiation of lens cells.