Gut microbiota have been shown to play a critical role in the maintenance of host health. Probiotics, which regulate gut microbiota balance, could serve as an effective alternative to antibiotic growth promoters. Since changes in the gastrointestinal tract, caused by a variety of different strains, groups and amounts of microorganisms, may be reflected in its histological structure, the aim of the present study was to examine the effects of rising doses of a mixed probiotic preparation on the structure and development of the small intestine of female turkeys. Eighty, three-day-old, healthy, female turkeys (Big-6 breed) were used in the current (16-week) study. The turkeys were randomly allocated to four weight-matched (59.70 ± 0.83 g) groups (n = 20), according to probiotic treatment dose (0, 107 cfu•g−1, 108 cfu•g−1 or 109 cfu•g−1, in 500 g•1000 kg−1) (cfu – a colony-forming unit). Three, non-genetically modified strains of probiotic cultures obtained from poultry, four bacterial and one yeast culture, were used. Histomorphometric analysis of the structure of the small intestinal wall of the duodenum and jejunum was performed. All probiotic doses used in the current study exerted a beneficial effect on the histological structure of the small intestine; however, the observed effect was dose and region dependent. Significant increases in villi height, crypt depth, villi and crypt width, mucosa thickness, epithelial height, enterocyte number, absorption surface and intestinal ganglia geometric indices were observed, specifically in the duodenum of birds receiving an intermediate dose of probiotic (108 cfu•g−1). The probiotic doses used in the current study differed significantly in their effect on the small intestine (P < 0.01), with the intermediate dose (108 cfu•g−1) significantly improving 58% of the parameters assessed, compared to the control. The duodenum was more susceptible to the favourable effects of the probiotic than the jejunum (56% v. 31% improvement in the parameters assessed) (P < 0.01). The weakest favourable effect was observed in the group that received the highest dose of probiotic.