Experiments are described which indicate that the degree to which ryegrasses, Loburn SPP., are damaged by Argentine stem weevil, Listronotusbonariensis (Kuschel), is affected bv three major factors. Ryegrasses with high levels of infection with andophytic fungi carry fewer stem weevil eggs and larvae and consequently suffer less damage. Some ryegrasses with low levels of endophyte infection, such as 'Grasslands Ruanui', suffer more damage when grown with high inputs of nitrogen, while others with the genotype Lolium multiflorum x perenne L., such as 'Grasslands Manawa' are more susceptible than those with the genotype Lolium perenne L. Keywords: Lolium spp., Listronotus bonariensis, endophvtic fungus, nitrogen, plant resistance, stem weevil