Some plant secondary metabolites as tannins have direct anthelminthic properties and may play a role in the control of nematodes in livestock. However, their great diversity in structural characteristics and different levels of content in plants are responsible for a highly variable response in anthelmintic activity, as measured in vitro. The aim of the present study was to assess the relationship between structure and anthelmintic activity using an in vitro assay. We used a series of purified tannins (from 65% to 100% of purity) characterized for their degree of polymerization (mDP), prodelphinidin/procyanidin ratio and cis/trans ratio by thiolytic degradation. Tannins diluted in two concentrations in water, epigallocatechin gallate, positive (ivermectin) and negative (water) controls were examined by the Larval Feeding Inhibition Assay (LFIA) with first stage larvae (L1) of the cattle nematode Cooperia oncophora in triplicates. All extracted condensed tannins, in particular sainfoin, white clover and pine tree inhibited the feeding of L1 at the high concentration. There was a trend towards lower inhibition with decreasing PD/PC ratio. However, despite black currant had 95 % of PD, it showed a low inhibition which could be related to a very low cis/trans ratio. Based on these preliminary findings it is difficult to define a relationship between the structural parameters and anthelmintic activity.