Our objective was to evaluate the effects of Acacia mearnsii bark extract (PAB) on methane (CH4) production in sheep infected with Trichostrongylus colubriformis and Haemonchus contortus. Twenty 10 month-old Santa Inês lambs were used in a 50-day trial. Four treatment groups were formed: two control groups uninfected, one without PAB (C-) (n=4) and one with PAB (C+) (n=4); and two infected groups, one without PAB (I-) (n=6) and another receiving PAB (I+) (n= 6). Animals were kept in individual pens for 40 days, and received ad libitum chopped tifton 85 hay (Cynodon spp.) and 210 g/animal/day of concentrate (PAB supplementation: 15 g/animal/day – in concentrate). Animals were reared indoors to avoid helminthic infections (faecal egg count (FEC) was monthly controlled). At day 0, infected groups were artificially infected with L3 larvae of T. colubriformis (7500)and H. contortus (5000). After 28 days post- infection, establishment of infections was confirmed by FEC: I-: 1150 + 530.1 and I+: 567 + 544.6 eggs/gram of feces. After 40 days of trial (during which other parameters were evaluated - data not shown), CH4 emissions in chambers were measured and variance analysis (proc GLM, SAS®) showed no PAB effects on methanogenesis (p > 0.05), however, both infected groups had higher CH4 emissions than the control groups: C-: 16.7 + 10.91; C+:23.7 + 12.88; I-: 43.1 + 6.37; I+: 40.6 + 6.02 (g CH4/kg dry matter intake) (p < 0.05). The methanogenic effect of parasitc infections should be explored in studies addressing sustainability of animal production.