R. meliloti carrying defined deletions that remove fix114 form Fix- nodules which are devoid of intracellular bacteria. Occasionally strains which carry these deletions form pink nodules which appear effective in contrast to the normal white ineffective nodules formed by strains carrying fix114 mutations. Bacteria isolated from these pink nodules retain the original deletion and form effective pink nodules when reinoculated onto alfalfa. It is hypothesized that these isolates carry second site mutations which enable the bacteria to overcome the symbiotic block associated with the fix114 mutation. In this work, five independent isolates were examined and were shown to carry second site mutations that suppress the symbiotic ineffectiveness completely on alfalfa and incompletely on sweet clover. The five independent second site revertants can be divided into two classes based on genetic data and on their sensitivity to detergents and both classes were localized to the chromosome of the wild type Rm1021. One such second site revertant allele, sfx-1, was cloned and localized to a large 18 kb BamHI fragment. / Thesis / Master of Science (MS)