Variation in growth of Medicago ciliaris was recorded across soils from 5 different regions in Tunisia that represented different soil types and climatic zones. In 4 of these soils (Mateur, Enfidha, Rhayet, and Soliman) this variation appeared to be related to the nodule number on the roots of the plants. With the exception of one isolate the rhizobia isolated from these nodules had 16S rRNA PCR-RFLP fingerprint patterns that were characteristic of Sinorhizobium medicae. Plant growth in the fifth soil (Jelma) was the poorest; plants had few nodules that yielded exclusively rhizobia with 16S rRNA fingerprint patterns characteristic of S. meliloti. In subsequent plant tests, S. medicae isolates formed effective nitrogen fixation symbioses with M. ciliaris, while S. meliloti formed small, white, ineffective nodules. Therefore, plant growth in Jelma soil was poor because only S. meliloti are present and this species is ineffective with M. ciliaris. In a co-inoculation experiment with M. ciliaris, S. medicae was more competitive for nodulation than S. meliloti, perhaps explaining why the majority of the isolates from Enfidha and Rhayet were S. medicae, since S. meliloti is present in these soils. However, it is not clear how the host influences rhizobia for nodulation by S. medicae in preference to S. meliloti when present.