"Black vomit" is a serious disease in chicks and is caused by some kinds of fish meal in the diet. However, the causative compound has not been identified. In the present study, a new substance, 2-amino-9-(4-imidazolyl)-7-azanonanoic acid, has been isolated from mackerel meal and named Gizzerosine. This compound caused severe gizzard erosion in chicks within a week when it was fed to them at the level of 2.2ppm in the diet. Gizzerosine is produced during fish meal manufacturing by the reaction between histidine and protein in the fish meat. The discovery of Gizzerosine raises a new problem in food chemistry and animal nutrition.