The treatment of wheat roots with 2,4-dichlorophenoxyacetic acid (2,4-D) led to the formation of tumour-like structures, para-nodules, which were readily colonized by cyanobacteria. The amount of cyanobacteria found on 2,4-D treated roots was 3.6 times higher than that found on untreated roots. Cyanobacteria penetrated the para- nodules by migrating in between loosely arranged cells that covered their surfaces or by penetrating the space at the junction of root and para-nodule. In plants treated with 2,4-D and co-cultivated with cyanobacteria in medium without combined nitrogen, the rate of acetylene reduction was three times that seen in untreated but colonized roots. Addition of 2,4-D itself did not change the rate of acetylene reduction in free-living culture of Nostoc sp. strain 2S9B. In plants treated with 2,4-D and co-cultivated with Nostoc sp. strain 2S9B in medium without combined nitrogen the nitrogen content of roots but not shoots was significantly increased.