Leucaena leucocephala is a tropical forage legume suggested as an alternative method to control gastrointestinal parasitism in ruminants. This study: (1) performed a bio-guided fractionation of an aqueous extract of L. leucocephala using the egg hatch assay (EHA) to identify the anthelmintic (AH)-like phytochemicals present in fresh leaves, and (2) assessed the ultrastructural damage to eggs of Cooperia spp. after incubation with the final fraction. Phytochemicals were isolated using silica gel columns and identified using high performance liquid chromatography and standards for comparison. The final fraction was evaluated using EHA at 0.06, 0.125, 0.250, 0.500 and 1.1 mg ml(-1). The lethal concentration to inhibit 50% of Cooperia spp. egg hatching (LC50) was calculated using a Probit analysis. Scanning and transmission electron microscopy revealed the ultrastructural changes present in Cooperia spp. eggs. Bio-guided isolation procedures led to the recognition of an active fraction (LlC1F3) mainly composed of quercetin (82.21%) and caffeic acid (13.42%) which inhibited 90.49 ± 2.8% of Cooperia spp. egg hatching (P