Species within the genus Calliandra are among the most widely used tree legumes in agroforestry systems. Calliandra calothyrsus has by far received the greatest International attention, especially in eastern and southern Africa. Under our nursery experiments, C. calothyrsus has consistently outperformed the other lesser-known Calliandra species such as C. houstoniana, C. juzepczukii and C. longepedicellata. In order to optimize biological nitrogen fixation (BNF) potential with the nodule-forming bacteria (Rhizobium), we undertook a series of experiments to investigate the relationships between both symbiotic partners (C. calothyrsus and Rhizobium) under nursery and field conditions. Firstly, under greenhouse conditions, we tested two C. calothyrsus provenances (Flores and San Ramon) inoculated with 16 different Rhizobium strains, and eleven C. calothyrsus provenances (Flores, Alotenango, Barillas, Santa Maria, Ixtapa, Union Juarez, Bonampak, San Ramon, Madiun, Georgesville and La Ceiba) inoculated with two Rhizobium strains (CCK12 and CCR20A). The inoculation effect on growth was positive but variable according to C. calothyrsus provenance and type of Rhizobium strain used. In the second experiment, we tested a selection of C. calothyrsus provenances (Georgesville, Union Juarez, Alotenango and Barillas) and Rhizobium strains (CCK13 and CCK12) in three soil types with different characteristics (e.g. clay, sandy, halfclay half-sandy). Our results showed that the symbiotic effectiveness was significantly affected by soil characteristics but not according to the provenances. In the field, 19 months after plantation, we were able to demonstrate a significant effect of the C. calothyrsus provenances on plant growth and nitrogen derived from atmosphere (Ndfa), whereas the inoculation effect became less significant on plant growth 11 months after plantation. In conclusion, our results demonstrated a strong interaction between C. calothyrsus provenance and Rhizobium strain,