SUMMARYThe production of pol ygalacturonase in associations of nodule bacteria and seedlings of leguminous plants was investigated. Plants and bacteria were combined in the following ways : ( a ) different plant species were combined with infective strains (isolated from the same cross inoculation group) as well as non-infective ones (from foreign groups) ; ( b ) host plant species with different susceptibilities were combined with the same bacterial strain; ( c ) bacterial strains with different infectivities, as measured by the number of infection sites, were combined with the same host species; ( d ) clover strains which had lost their infectivity, as well as transformed and again infective subcultures of these strains, were tested on clover plants. The results indicate that infection of the seedlings was strongly correlated with the production of polygalacturonase. The conclusion is drawn that polygalacturonase plays an important part in the infection process. This function is thought to be a weakening of the cell wall of the root hair which would facilitate the bacterial invasion. The possible role also of indolyl-$-acetic acid in the infection process is discussed.