Poultry, like mammals and other birds, develop fever when exposed to compounds from gram-negative bacteria. Mammals also develop fever when exposed to the constituents of viruses or gram-positive bacteria, and the fevers stimulated by these different pathogenic classes have discrete characteristics. It is not known whether birds develop fever when infected by viruses or gram-positive bacteria. Therefore, we injected Pekin ducks with muramyl dipeptide, the cell walls of heat-killed Staphylococcus aureus, or the viral mimic polyinosinic: polycytidylic acid and monitored their body temperature (T-b). For comparative purposes we also injected a group of ducks with lipopolysaccharide, the only known pyrogen in birds. We then compared the T-b invoked by each injection with the T-b after an injection of saline. Muramyl dipeptide did not affect T-b. The cell walls of heat-killed S. aureus invoked long-lasting, dose-dependent fevers with relatively low magnitudes. Polyinosinic: polycytidylic acid invoked dose-dependent fevers with high febrile peaks. Fever is a well-known clinical sign of infection in mammals, and the results of this study indicate that the pattern of increase in T-b could serve as an indicator for diverse pathogenic diseases in birds.