Examination of 22 Proteus morganii strains revealed that each possessed histidine decarboxylase activity. Strain GRMO 6 produced maximal activity (nanomoles of CO2 released/mg cells/h) when grown at ambient temperature (24 C) and at acid pH levels that tended to inhibit growth. Minimal activity was present when the culture was grown at pH 8.5. Histidine decarboxylase activity decreased as the age of the culture increased. Maximal reaction rates occurred at 37 C and pH 6.5. Rapid enzyme and histamine formation occurred in tuna fillets inoculated with P. morganii and stored at 24 and 30 C. Histamine levels reached 520 mg/100 g and 608 mg/100 g at 24 and 30 C, respectively. Little enzyme was produced in the inoculated fillets stored at 15 C and in the uninoculated control fillets.