Aims: To evaluate the effect of nitrofuran derivatives furazolidone (Fz) and nitrofurantoin (No on Salmonella enterica serovar Enteritidis PT4 in vitro, with regard to cell growth, morphology and ultrastructure. Methods and Results: The effects of Fz on the growth rates of Fz resistant (Fz(R)) and sensitive (Fz(S)) strains were assessed by viable counts. Over 24 h incubation, concentrations of <1 μg ml(-1) of Fz were bacteriostatic to the Fz(S) strain. The Fz(R) strain tolerated concentrations up to 16 μg ml(-1) before cell numbers diminished over the same time period. The effect on the growth rate of the Fz(S) strain after 1 h exposure to supra-inhibitory concentrations of Fz, gave a maximum response at 32X minimum inhibitory concentration (MIC) of 4.5 h. Effects on the ultrastructure of bacterial cells by scanning electron and transmission microscopy, and DNA-specific staining with DAPI of the Fz(S) strain exposed to nitrofurans were studied. Abnormalities such as extensive filamentation with sparse, sporadic nucleotide distribution and evidence of extrusions in the cell envelope in the form of blebs were evident. Conclusions: Nitrofurans exert their bactericidal effect on Salmonella by inducing extensive structural alteration after exposure at sub- or suprainhibitory concentrations, involving inhibition of cell division because of the activated drug causing an intercalating type of binding in DNA. Significance and Impact of the Study: These results demonstrate the in vitro activity of the nitrofuran derivatives, furazolidone and nitrofurantoin on Salmonella, defining, the pharmacodynamics and physical nature of their action as therapeutic agents.