Utilization of several iron sources available from the host was investigated in different strains of Vibrio anguillarum. We tested the ability to use transferrins, heme, hemoglobin, and haptoglobin-hemoglobin as iron sources in strains of V. anguillarum possessing different iron uptake systems mediated by siderophores. Only the wild-type pathogenic strains with an intact siderophore-mediated iron transport system were able to obtain iron from transferrins. None of the low-virulence derivatives lacking siderophore production could grow in the presence of transferrins. However. all strains, wild-type and iron-deficient derivatives, could utilize heme, hemoglobin, and haptoglobin-hemoglobin as iron sources when added to iron-deficient media. The ability to grow in fish serum was also evaluated. Although only wild-type strains could grow in fresh serum, derivative strains lacking siderophore production also were able to grow when serum was heat inactivated or when a utilizable siderophore was present in serum. The results indicate that besides the siderophore-mediated mechanism, V. anguillarum can also obtain iron from other sources presumably available from the host, although its importance for growth in vivo is so far unknown.