To improve Neotyphodium endophyte-mediated resistance to black cutworm Agrotis ipsilon (Hufnagel) (BCW), a series of experiments was conducted by using several different cultivars of tall fescue, Schedonorus arundinaceus (Schreb.) Dumort. in combination with several different haplotypes of the endophyte Neotyphodium coenophialum (Morgan-Jones & Gams) (plant cultivar × endophyte haplotype = plant line), each producing unique alkaloid profiles. BCW settling response, survival at 5 and 10 d, and larval biomass varied significantly among plant lines. In general, greater variation BCW performance was observed within a single plant cultivar infected with different endophyte haplotypes than among different plant cultivars infected with the same endophyte haplotype, but comparisons among the former were far more numerous. Although five endophyte-mediated alkaloids representing three alkaloid classes were quantified in the plants, the pyrrolizidine alkaloid N-acetyl norloline was consistently the single best predictor of BCW performance. BCW settling response, 5-d survival, and 10-d survival decreased as levels of the alkaloid N-acetyl norloline increased. The same three response variables also decreased with increasing levels of peramine, but increased with increasing levels of ergovaline. Minor variation in endophyte infection levels occurring among infected plant lines had no significant influence on BCW performance. Results indicate a potentially important role for N-acetyl norloline and peramine in providing resistance to black cutworm whereas ergovaline appears to be much less important. Therefore, endophyte haplotypes expressing high levels of N-acetyl norloline and peramine may be of particular importance for developing 'friendly' endophyte-enhanced turf and pasture grasses that resist challenging lepidopteran pests, although remaining safe for wildlife and grazing mammals.