Associative nitrogen-fixation is one mechanism by which plants can meet a portion of their nitrogen (N) needs in N deficient environments. Eastern gamagrass [Tripsacum dactyloides (L.) L.] grows under conditions conducive for associative N-fixation. Our objective was to evaluate eastern gamagrass roots for nitrogenase activity and associated N-fixing bacteria, and compare its nitrogenase activity to switchgrass (Panicum virgatum L.). Soil and root samples were collected from unfertilized stands of ''PMK-24'' eastern gamagrass at 3 locations in northern Missouri and nitrogenase activity measured using the acetylene reduction method. Eastern gamagrass roots supported greater nitrogenase activity when colonized by indigenous bacteria, compared to roots inoculated with Klebsiella pneumoniae or surface sterilized. Eastern gamagrass roots colonized by indigenous bacteria had 17-46 fold greater nitrogenase activity than switchgrass roots. Bacterial composition on eastern gamagrass roots was dominated by fluorescent pseudomonads and Pseudomonas cepacia, with lower populations of other bacteria. Bacterial composition from incubation tubes showing nitrogenase activity was similar to that found on the roots, except that the diazotrophic bacterium Azospirillum braziliense was isolated from 1 of 3 sites. The potential for associative N-fixation with eastern gamagrass warrants further study and in situ measurement to quantify rates of N fixation.