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To confirm the hypothesis that Salmonella entericasubspecies entericaserovar (S.) Infantis has higher basic reproductive rates in chicks compared with other Salmonellaserovars, 1-day-old specific-pathogen-free chicks (n= 8) were challenged simultaneously with S.Infantis and S.Typhimurium per os. Challenged chicks (Group A) were then housed with non-infected chicks (Group B, n= 4) for 6 days (from 2 to 8 days of age). Group B birds were then housed with other non-infected birds (Group C, n= 4), which were then transferred to cages containing a further group of untreated chicks (Group D, n= 2). A control group consisting of four non-infected chicks was used for comparison. All chickens were humanely sacrificed at 18 days of age, and Salmonellafrom bowel and liver samples were enumerated. Both serovars were isolated from all groups except the control group. S.Typhimurium was isolated at a greater frequency than S.Infantis from the bowel samples of chicks from Groups B, C and D, while no differences in colonisation rates were observed between the two serovars in liver samples from Groups B, C and D. S.Typhimurium, but not S.Infantis, was immunohistochemically detected in the lamina propria of the cecum and rectum in five birds of Group A. Despite the competitive administration, neither of the two serovars completely excluded the other, and no differences were observed in basic reproductive rates between the two serovars. These findings, together with data from previous studies, suggest that the initial quantitative domination of S.Infantis in chicken flocks may explain why this serovar is predominant in broiler chickens.