It had previously been reported that the M. truncatula plants showed differential nitrogen fixation efficiency when inoculated with different rhizobial strains [32-35]. However, to our knowledge, natural variation in nodula- tion specificity (i.e., Nod + vs. Nod- phenotypes) has not been well-documented. To gain a better understanding of the genetic mechanisms underlying symbiosis specifi- city in the M. truncatula-Sinorhizobium interaction, we screened a core collection of 31 M. truncatula genotypes using the S. meliloti strains NGR34, NGR247, and Rm41. These plant genotypes capture a wide range of genetic di- versity present in natural populations of M. truncatula [36]. This experiment revealed tremendous variation in nodulation capacity and nitrogen fixation specificity be- tween different genotype-rhizobial combinations (Table 1). In particular, this screen revealed that Rm41 was unable to nodulate the plant genotypes F83005.5 and Turkey (Figure 1), while the same plant genotypes nodulated nor- mally with other S. meliloti strains. Thus, we postulate that there exist host genes that control strain-specific nodula- tion in M. truncatula. For genetic analysis of the nodula- tion specificity in this system, we chose to focus on F83005.5 because Turkey was not compatible when crossed with several other M. truncatula genotypes.