Abstract: An investigation was made at a double-rice paddy field set up in 1982 in the Red Soil Field Experimental Station, CAAS, China to study the species and biomass of weeds growing during the rice (Oryza sativa) growing seasons. Over a 30-year period, organic, chemical, and a mixed organic and inorganic fertiliser were applied with the same rates of nitrogen (N), phosphorus (P), and potassium (K). Compared with other treatments, long-term application of mixed fertiliser resulted in the least number of weed species in both rice growing seasons. Organic fertiliser application resulted in the greatest number of weed species in the early rice growing season, while treatment without fertiliser application resulted in the greatest number in the late growing season: 4.4% and 39.3% higher than the former two respectively. Long-term application of organic fertiliser resulted in the highest biomass of weeds in both seasons, 10.7%, 4.5% and 37.7% higher than the applications of mixed, chemical, and no fertiliser respectively. Long-term application of organic fertiliser resulted in the highest biomass of all weeds and floating weeds in the early season, the treatment without fertiliser application resulted in the highest biomass of both in the late season, while mixed fertiliser application resulted in the highest biomass of floating weeds in both seasons. Soil alkali-hydrolyzable N and available P were positively correlated with the dry mass of weeds (r=0.465, P<0.05 and r=0.517, P<0.05, respectively), and of wet weeds (r=0.482, P<0.05 and r=0.542, P<0.05, respectively). Dry mass of floating weeds was positively correlated with soil available P (r=0.666, P<0.01), and was negatively correlated with soil pH (r=0.516, P<0.01). Soil available P, under the co-effects of soil pH, alkali-hydrolyzable N and other factors, had an indirect effect on the dry mass of weeds. If effective control of the growth of wet and floating weeds in red soil paddy of south China is to be attempted by achieving a beneficial dynamic equilibrium between the weed species and total biomasses, the best way is to adopt various fertilisation measures to maintain proper alkali-hydrolyzable N and available P contents and soil pH.