The relationship between the growth of weeds, namely (1) Heleocharis acicularis, R. Br., (2) Cyperus nipponicus, Franch. et. Sav., (3) Monochoria vaginalis, Presl. var. plantaginea, Solms-Lamb., (4) Rotala indica, Koehne., (5) Lindernia pyxidaria, A11., (6) Lindernia augustifolia, Wetts., (7) Spirendora polyrhi.a, Schleid., (8) Lemna pancicostata, Hegelm. and (9) Potamcgeton polygonifolius, Pourr., and the reaction of soil and the stagnant water in the paddy-field was investigated ; and found that in case of soil, all these weeds grew more or less within pH 3.3-9.2,and (1) weed was most resistant thru all pHs ; within pH 5.0-6.7 all the weeds grew well, while in pH 7.3-7.8 the growth was less. In the stagnant water, (7), (8) and (9) weeds were tested, and found that pH 3.2 and 9.4 as the limiting reaction, pH 6.5-7.5 as the optimum. Further (7) and (8) weeds were compared as to the growth rate in different pHs and found a big difference. From these results, it is reported that to prevent the growth of these weeds by adjusting pH in the soil and the stagnant water is difficult, since the effect on the rice plants should be considered at the same time. The manure value of these weeds will be investigated in future.