IN DECEMBER 1998, the Chester County Health Department (CCHD) in Pennsylvania received reports of four cases of scombroid fish poisoning among patrons at a local restaurant. This report summarizes the investigation of these cases by CCHD, the Pennsylvania Department of Agriculture (PDAg), and the Pennsylvania Department of Health (PDOH). Findings from this investigation suggest that initial processes that are not regulated by the Food and Drug Administration (FDA) (i.e., from hooking the fish to unloading the fish on the dock) may permit scombrotoxin formation.