Abstract Thirty seven diazotrophs from nine genera were examined for nitrogenase activity when grown on various carbon substrates, viz. straw, both in pure culture and in co-culture with Cellulomonas sp. CS1-17, the straw breakdown products cellobiose and glucose, a component of hemi-cellulose (xylan) and the substituted cellulose, carboxymethyl cellulose (CMC). Strains of Azospirillum spp, Azotobacter spp, Azomonas spp, Beijerinckia spp, Pseudomonas spp, Bacillus spp and Bradyrhizobium spp were all capable of utilising the compounds released from straw by the action of cellulases produced by Cellulomonas sp. and producing nitrogenase activity which lasted the duration of the experiment (15 or 18 days). Some strains of Azospirillium, Azotobacter, Azomonas and Bradyrhizobium were capable of short periods of nitrogenase activity on straw in pure culture. This indicated these organisms utilised components of the straw prior to degradation by cellulolytic organisms, enabling them to establish rapidly. This would be an important consideration in selecting strains for use in inoculants. For all strains, nitrogenase activity on CMC was weak or absent, while nitrogenase activity on xylan and cellobiose was variable within genera. The significance of these results for the development of an inoculum to increase nitrogen fixation in straw-amended soil is discussed.