Machaerothrix tsushimensis Yasumatsu exhibits several previously undescribed characteristics in its behavior. The nest is composed of barrel-shaped cells and an outer covering. Before hunting females recondition old cells for reuse, by scraping dried mud from the nest surface with their mandibles, adding water, and working it into a soft ball, which is then daubed with the dorsal tip of the gaster. They capture salticid spiders and store them in the clay nests. The female amputates part or all of the spider's legs and walks forward, straddling the spider, holding it venter up by the spinnerets. Several females live together on the same nest for some period. They are usually tolerant of one another, though dominance interactions are found among them. The dominant female exhibits a full sequence of nesting behavior, including cell provisioning, but the subordinate tends to spend much time in resting on the nest. There is evidence that some of the subordinates initiate nesting activities on the nest where they have cohabited with the dominant, after the disappearance of the latter, or construct a new nest independently after cohabiting with the dominant. Prey stealing and cooperative nest construction and defence were observed. Nesting and social behaviors of this species are compared with those of other primitively social pompilids and the possible position of this species in the evolution of social behavior is discussed.