Some weed biological control agents are released despite predictions that they might damage nontarget host plants in the field. Based on prerelease laboratory tests it was predicted that Neurostrota gunniella, an agent released in Australia against Mimosa pigra, may occasionally use Neptunia spp. as hosts. However, due to high larval mortality on these species, N. gunniella was not expected to persist on Neptunia spp., nor have a significant effect on them. N. gunniella has established widely and is now abundant on the target weed, which grows sympatrically with at least one species of Neptunia. We investigated the nontarget attack of Neptunia major in the field, and the effect of that attack on plant growth and reproduction. Although an average of 61% of N. major plants growing adjacent to M. pigra thickets had evidence of N. gunniella attack, the intensity of attack was relatively low. At sites where M. pigra was not present, use of N. major plants by N. gunniella was noticeably reduced or absent. From glasshouse experiments, we determined that N. gunniella attack causes increased stem tip death and hinders plant growth in both plantlets and mature plants. At the highest level of attack recorded on N. major in the field, tip death increases by 54% and plant height would decrease by 6%. The effect of N. gunniella on the reproductive output of N. major could not be clearly resolved, but is likely to be small. Our findings support the predictions made during prerelease studies of N. gunniella. (c) 2007 Elsevier Inc. All rights reserved.