The present study investigated the effect of a fibrolytic probiotic using bacteria isolated from the rumen of the North American moose (Alces alces). Bacteria from moose were chosen for their ability to digest a variety of plant polysaccharides and lignin. Twenty 5-d old lambs (mean 5.9 ± 0.2 kg) were divided into probiotic and control groups, with probiotic receiving 5 bacterial isolates orally through weaning (9 wk), after which the lambs were put on pasture. Neither weight gain nor wool quality was improved in lambs given a probiotic, however, dietary efficiency was increased as evidenced by the reduced feed intake (and rearing costs) without a loss to weight gain. Total VFAs were not significantly different between groups. Acetate, propionate, butyrate, and ethanol were significantly (P < 0.05) higher in experimental lambs at certain weeks. The acetate to propionate ratio was statistically lower in the experimental group at wk 9, 11, and 15, which was previously shown to indicate increased dietary efficiency. Sampling coverage decreased over time, while Shannon, Inverse Simpson, CHAO, and ACE increased over time, which is a function of the increasing diversity of microbiota as the rumen develops. The experimental group had a higher diversity at the beginning of the experiment. Bacteroidetes was the most prevalent phylum (38–73% of total sequences) in both groups for the duration of the study, with the exception of the first sampling of the control group. Firmicutes was the second most prevalent (23–59%). In the control group, Bacteroidetes increased while Firmicutes decreased for the first 3 sampling points, while the experimental had a general trend of decreasing Bacteroidetes and increasing Firmicutes over time. Prevotella was the most prevalent genus, followed by Butyrivibrio and Ruminococcus. Protozoal densities increased over time and stabilized, but methanogen densities varied greatly in the first 6 mo of life for lambs. This is likely due to the changing diet and bacterial populations in the rumen.