Most strains ofPseudomonas aeruginosaisolated from patients were lethal to burned mice but rats were relatively resistant and died of certain strains only. In burned rats, silver sulfadiazine (1.0%) applied daily beginning 24 hours postseeding with ratvirulent organisms resulted in 11.2% mortality compared to 12.5% mortality after treatment with mafenide. With certain strains, death occurred after treatment with mafenide but not with silver sulfadiazine. The effect of silver sulfadiazine was dose-related. In mice seeded with human-virulent strains (which were not lethal to rats) or strains lethal to both species the mortality was less than 20% with silver sulfadiazine and 83% with mafenide. Silver sulfadiazine proved effective in both rats and mice infected with virulent strains of Pseudomonas; mafenide was equally effective in rats but relatively ineffective in mice.