The role of the intestinal tract in Vibrio anguillarum infection of turbot, Scophthalmus maximus (L.), fingerlings was investigated in two in vivo models and the possible mechanisms involved were studied in vitro. Viable V. anguillarum cells were detected in spleens from more than 50% of the fish administered the pathogen orally or rectally, suggesting that the intestinal tract is a portal of entry for V anguillarum. In transmission electron micrographs, V. anguillarumlike cells were seen close to the rectal epithelium, suggesting penetration of the mucus layer, but no epithelial cell penetration or endocytosis was evident. Attachment to intact turbot intestines was investigated, and 80% or mote of the bacterial cells still remained attached after serial washings. A significantly higher number of cells attached to rectal segments than to the other intestinal segments. In vitro, V. anguillarum cells did not adhere specifically to intestinal mucus, but rather accumulated close to intestinal mucus interfaces and subsequently penetrated them. It is proposed that the intestinal tract of turbot is a portal of entry for V. anguillarum and that the cells penetrate the intestinal mucus overlaying the epithelial cells.