Chamaesyce maculata is a prostrate annual weed of Euphorbiaceae, and produces seeds from summer to autumn in western Japan. It has two seasonal different modes of seed dispersal. The seeds produced in summer are positioned further from the parents by automatic mechanical seed dispersal (autochory). On the other hand, no autochory occurs in the seeds produced in autumn, but some seeds are dispersed by ants. We had observed two ant species, Tetramorium tsushimae and Pheidole noda, carrying the seeds of C. maculata although 18 ant species in total visited the plants at 50 sites. In this study, we performed a laboratory experiment to investigate the state of C. maculata’ seeds in nests of T. tsushimae and P. noda. The two ants carried the seeds into their nests. Pheidole noda hardly carried the seeds out of the nest, and consumed the seeds in the nest. On the other hand, T. tsushimae consumed only seed coat, and carried the seeds out of the nest. Next we observed the ant behaviour against the filter paper with the water-extraction of the seed coat. Pheidole noda did not react to the filter paper, but T. tsushimae carried the filter paper into the nest. It is likely that C. maculata in Japan depends on a specific ant partner in seed dispersal.