Entomopathogenic nematodes (EPN) are currently marketed worldwide for use in inundative biological control, where the applied natural enemy population (rather than its offspring) is expected to reduce insect numbers. Unlike classical biological control, in inundative control natural enemy establishment is not crucial in order to achieve pest suppression. Field trials in Irish forestry provided the opportunity to test predictions regarding the establishment of two exotic (Steinernema carpocapsae and Heterorhabditis megidis) and two indigenous (Steinernema feltiae and Heterorhabditis downesi) species. Nematodes were inundatively applied to pine stumps to control populations of pine weevil, Hylobius abietis, on three clearcut sites, and their persistence and spread monitored for up to five years. All species were recovered three years after application but only S. feltiae was recovered in years 4 and 5. Limited horizontal dispersal to 20 cm (but not 100 cm) was observed, but the majority of nematodes were recovered close to the area of application. Steinernema feltiae was also recovered from nearby stumps to which it had not been applied, indicating possible phoretic dispersal by weevils or other stump-associated fauna. EPN were not recovered from stumps outside the treated area, suggesting that such dispersal is quite localized. Two strains of S. feltiae (Irish and exotic) were applied. Amplified fragment length polymorphism (AFLP) analysis on 11 populations isolated from soil four years later showed that all had a much closer affinity to the applied Irish strain, suggesting persistence of this genotype and extinction of the exotic one. Some strains were clustered close together, and this is interpreted in the light of possible population genetic scenarios. The findings from the field study confirm predictions based on background knowledge of the species and demonstrate the importance of mediumterm studies, as a 3-year study would have overestimated the risk of establishment of exotic species. Short-term persistence and spread of S. carpocapsae, S. feltiae, and H. downesi was also studied in pine forest mesocosms. Similar trends to field results, such as limited horizontal dispersal, even vertical distribution, and more abundant recovery of S. feltiae than of other species, point to the utility of mesocosm studies as a predictive tool.