The objective of this work over the last 3 years was to identify maize–endophyte associations with increased plant productivity compared with uninoculated controls. We have used a collection of endophytes isolated by several groups. The experiments were done under field and greenhouse conditions in the presence or absence of added fixed nitrogen (N). Significant yield enhancements of N-fertilized maize were obtained with bacterial endophytes that we have isolated from N-efficient lines of maize (such as Klebsiella pneumoniae 342) or switchgrass (Pantoea agglomerans P101 and P102). Several other strains from other groups were also tested with our best yield enhancements from two Brazilian strains, Gluconacetobacter diazotrophicus PAl5 andHerbaspirillum seropedicae Z152. Field experiments in Wisconsin were conducted in 1998, 1999 and 2000 and in an additional four states (Illinois, Iowa, Indiana and Nebraska) in 2000, with a minimum of two elite lines of maize at each site, each year. No strains were capable of relieving the N-deficiency symptoms of unfertilized maize in either the field or the greenhouse.