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A . S A N T R A A N D S . A . K A R I M . 2002.Aims: To assess the effect of presence or absence of rumen protozoa on fermentation characteristics and enzyme profile in growing lambs. Methods and Results: Weaner lambs (G1, G2, G3, G4, G5 and G6 groups) were defaunated by oral administration of sodium laurel sulphate (at 8 g 100 kg )1 body weight). The lambs of G4, G5 and G6 groups were refaunated. The roughage and concentrate ratio in the diet of G1 and G4, G2 and G5, and G3 and G6 were 50:50 (R1), 65:35 (R2) and 80:20 (R3), respectively. Daily dry matter intake was similar in defaunated and faunated lambs. However, digestibility of organic matter (OM), cellulose and gross energy were lower in defaunated lambs while crude protein (CP) digestibility was similar in both defaunated and faunated lambs. The rumen pH and NH 3 -N were lower (P < 0AE01) while TVFA, total-N and TCA-ppt-N were higher (P < 0AE01), in defaunated lambs. Ruminal activity of carboxymethyl cellulase was lower (P < 0AE01) in defaunated lambs and amylase, xylanase, protease and urease were similar in faunated and defaunated lambs. Nutrient utilization, rumen metabolites and ciliate protozoal count were higher, whereas digestibility of fibre fractions was lower in high rather than low concentrate fed lambs. The rumen protozoa present before defaunation were B-type and the protozoa which re-established on refaunation were also B-type. Conclusions: Absence of ciliate protozoa decreased nutrient digestibility and increased ruminal TVFA and total-N with lower NH 3 -N concentration, indicating better energy and protein utilization in defaunated lambs. Significance and Impact of the Study: Defaunation improved energy and protein utilization in lambs.