Short-and long-term persistence of pathogenic (i.e., tumor forming) agrobacteria in soil was investigated in six nursery plots with a history of high crown gall incidence. No pathogenic Agrobacterium strains were isolated in soil samples taken in fall and winter in any plots, but such strains were isolated from both bulk soils and weed rhizospheres (over 0.5 ؋ 10 5 pathogenic CFU/g of bulk soil or rhizosphere) in three out of six plots in spring and summer. PCR amplifications of a vir sequence from DNA extracted from soil confirmed the presence of Ti plasmids in summer and their absence in fall and winter. The results indicate that strains that harbor a Ti plasmid had an unforeseen positive fitness versus Ti plasmid-free strains in soil and rhizosphere in spring and summer in spite of the apparent absence of tumor, and hence of opines. The gain of fitness occurred during a bloom of all cultivable agrobacteria observed only in conducive soils. An evolution of the pathogenic population was recorded during a 4-year period in one particularly conducive soil. In 1990, the pathogenic population in this soil consisted of only biovar 1 strains harboring both octopine-and nopaline-type Ti plasmids. In 1994, it consisted of only nopaline-type Ti plasmids equally distributed among biovar 1 and 2 strains. These results suggest that nopaline-type Ti plasmids conferred a better survival ability than octopinetype Ti plasmids to biovar 2 agrobacteria under the present field conditions. Agrobacterium spp. are a genus in the family Rhizobiaceae. Strains of Agrobacterium harboring a tumor-inducing (Ti) plasmid cause the plant disease crown gall, which is characterized by uncontrolled proliferation of plant tissues (e.g., tumors). In the course of the infection process, a region of the Ti plasmid, the T-DNA, is transferred to and integrated into the plant genome (13, 34). In the transformed plant cell, T-DNA genes encode enzymes responsible for the uncontrolled synthesis of the plant hormones auxin and cytokinin, which account for the appearance of plant tissue hyperplasia. Other T-DNA genes encode enzymes responsible for the biosynthesis of particular compounds by the transformed plant cells, called opines. Opines are utilized as energy sources by strains harboring a Ti plasmid. Some opines induce the conjugal transfer of the Ti plasmids. Opines are relatively specific niches for strains harboring Ti plasmids, and tumors that produce opines are favorable habitats for both the proliferation of agrobacteria and the conjugal spread of Ti plasmids (10).Ti plasmids are large genetic elements, representing about 5% of the agrobacterial genome (1). As a consequence, they might negatively affect the fitness of Agrobacterium in the absence of opines. When population dynamics were monitored in the presence of opines, the closely related Ri plasmid was found to improve the growth and thus the fitness of a strain harboring this plasmid over those of a plasmid-free strain. Conversely, in the absence of opine, Ri and related plasmids hampered t...