When oncomiracidia of Neobenedenia girellae (Monogenea, Capsalidae) were incubated in wells with lyophilized extracts of fish skin epithelia on the bottom, some attached to the well bottom with the haptor unfolded and shed the ciliated epidermal cells. Based on these morphological changes in oncomiracidia, we developed a new assay method to examine the attachment-inducing capacities of various fish extracts for oncomiracidia. Attachment-inducing capacities were found only in extracts of fish skin epithelium and not in other fish extracts. No significant difference in capacities was observed among extracts of skin epithelia of 4 fish species. Wheat germ lectin and concanavalin A suppressed capacities in extracts of skin epithelia of Japanese flounder and yellowtail, respectively. Suppressed capacities were recovered by adding sugars that bound specifically to these lectins. These results indicate that some sugar-related chemical substances that exist specifically in fish epithelium induce the attachment of N. girellae oncomiracidia.