The water extract of the stem bark of Sacoglottis gabonensis was evaluated for its preliminary acute toxicity and anthelmintic efficacy against gastro-intestinal nematodes of small ruminants and mice in both in vitro and in vivo studies. Intra-peritoneal administration of doses ranging from 400 to 3200 mg/kg of the aqueous stem bark extract produced varying degrees of toxicity manifested as depression, drowsiness and unsteady gait, paralysis of the hind limbs, dyspnoea, coma and death. The pathological lesions noted at necropsy were mainly congestion and edema of the lungs, bronchi and bronchioles and hepatomegally with focal necrosis of liver cells. The severity of the clinical symptoms and pathological lesions were dose-related. In the in vitro study, the extract significantly (P<0.05) reduced the hatching of strongyline nematode eggs from naturally infected small ruminants. The 100 mg/ml concentration of the extract produced the highest (94.4%) inhibition on nematode egg hatch and the result was comparable to similar effect produced by either levamisole (100% at 15 mg/ml) or albendazole (99.7% at 6.25 mg/ml). In rats experimentally infected with Heligmosomoides polygyrus, treatment with the S. gabonensis stem bark aqueous extract significantly (P<0.05) reduced adult worm burden and completely inhibited faecal egg output 5 days post treatment.