Four studies were conducted to determine the survival of Megasphaera elsdenii NCIMB 41125 in the gastro intestinal tract (GIT) of Holstein calves after an oral dose, and evaluate its effects on ruminal fermentation, rumen development and performance. The objective of the first study was to investigate the survival of the bacteria in the GIT and evaluate effects on rumen fermentation and blood metabolites. The second study evaluated the effects and dosing time on intake, growth and diarrhoea in a conventional calf growth system. The third study evaluated the pre- (d 1-56) and post-weaning (d 56-70) effects of the bacteria in an accelerated growth system on intake, growth and blood beta-hydroxybutyrate (BHBA). The fourth study evaluated the effects on rumen development and fermentation. In experiment one, a higher cell count of M. elsdenii (24 h after dosing) and M. elsdenii NCIMB 41125 (24 and 72 h after dosing) were found in the rumen and the colon after dosing on day 7 and 14. An increase in the proportion of butyrate was observed 72 hours later. In experiment 2, measured for 42 days after birth, dry matter and water intakes were increased (P < 0.05) after dosing with M. elsdenii NCIMB 41125 on days 7 and 14 when compared to control calves. Feed efficiency, however, was not affected by dosing. The average numbers of days that diarrhoea was observed were 18 and 4.5 % less in calves dosed on d 7 and d 14, and 11.4 % higher in those dosed on d 21 compared to control calves. In the third study, dosed calves had higher starter DMI compared to control calves (P < 0.01). The opposite was true for milk intake as dosed calves consumed less milk (P = 0.01) than control calves. The total DMI, energy intake and ADG were not affected (P > 0.05), but dosed calves were 5.8 kg heavier (P = 0.01) at weaning and had greater gain:feed ratio (P < 0.01) compared to control calves. The plasma BHBA concentration after dosing was greater for dosed calves (P = 0.02) compared to control calves. After weaning, dosed calves consumed more (P < 0.05) feed and energy, gained 0.37 kg more weight per day (P < 0.01), and weighed 11.6 kg more (P < 0.001) at the end of the study compared to control calves. In the last experiment, dosed calves had a higher reticulo-rumen weight (P = 0.01), papillae width (P < 0.001) and papillae density (P = 0.02) compared to control calves. The rumen wall thickness and papillae length were not affected by dosing. Total VFA s, acetate and propionate were also not affected, but butyrate concentration was higher in dosed compared to control calves (P < 0.04). These results indicate that M. elsdenii NCIMB 411 can colonize and establish in the GIT of pre-weaned calves after a single dose of the bacteria. The increased starter intake, plasma BHBA, and improved rumen fermentation and development in dosed calves suggest greater metabolic activity of the rumen epithelium. Benefit of dosing on feed efficiency was observed on calves in accelerate growth and early weaning.