The percentage of spherical colonies from the trichomes of Nostoc sphaeroides reached 62-73% after 16 days with 50 and 250 mu M P, but only10-15% at 0.5 and 5 mu M P. During colony formation from microcolonies to macrocolonies, the growth rates were 95, 206 and 244% higher, respectively at 5, 50 and 250 mu M P than that at 0.5 mu M P. The light-saturated photosynthetic rate, maximum electron transport rate and light-limited photosynthetic efficiency at 0.5 mu M P decreased, respectively, by 45, 51 and 32% than those at 250 mu M P. These indicated that the colony development, growth and photosynthetic capacities were restricted at low P level, suggesting that P might be an important factor limiting the productivity and distribution of N. sphaeroides in the field.