Abstract: To determine the distribution characteristics of rhizobium in alfalfa plants and seeds, the distribution and abundance of endogenous rhizobium in different parts and seed of alfalfa at different growth stages were analyzed. The result indicates significant spatial and temporal heterogeneity in endogenous rhizobium abundance in alfalfa plant. For the spatial distribution, rhizobium abundance gradually increases with source-sink transportation trend of plant photosynthate. Most endogenous rhizobia are distributed in alfalfa root, mainly in the hair roots. In main root, more endogenous rhizobia exist in the epidermis and cortex areas compared with stele. Regarding the above-ground plant parts, endogenous rhizobia are mainly distributed in flower buds at vegetative stage 8.6~9.6×10³cfu·g^-1(FW), and in ovary walls of pistils at budding and floral stage. For pod-bearing stage, endogenous rhizobia are mainly in the pericarp of legumes (1.07×10³cfu·pod^-1), and in newborn seeds at mature stage. Rhizobium abundance in stalk is less than 2×10²cfu·g^-1(FW) at vegetative and pod-bearing stages, which vanishes at budding and floral stages. Rhizobia are not found in leaves. Temporally, rhizobium abundance in root and pericarp is obviously higher at pod-bearing stage than any other growth stage. Rhizobium abundance in various floral organs (except pedicle) rapidly increases after pollination. During ovary-to-legume germinative process, rhizobium abundance in ovary wall and ovule increases logarithmically. Endogenous rhizobia are found in ovule only after fertilization, and rhizobium abundance in young seeds is higher than in fertilized ovule. This implies that endogenous rhizobia can be transported and colonized in early germinated seedlings. Rhizobium abundance in mature seeds of the two alfalfa varieties stored for 120 d after harvest are 131.46 (“Longdong”) and 11.76 (“Eureka”) times higher than in newly harvested seeds. This suggests continuous proliferation of rhizobia after seeds are infested.