The DNA content was employed as an indicator of algae biomass to improve the cellular RNA content method to estimate the in situ growth rate of Microcystis in a hypertrophic lake (Lake Taihu, China). Microcystis aeruginosa was cultured in BG-11 and M-11 mediums, respectively, and a significant relationship between cell density and DNA concentration of Microcystis was found. Then a significant logarithmic relationship between Microcystis growth rate and RNA/DNA ratio was established to measure growth rate of Microcystis by analysis of RNA and DNA content. The growth rates calculated by RNA/DNA method were in good agreement with that calculated by the changes of cell density in the whole water column respected with time. The RNA/DNA method was applied in Lake Taihu in summer (1st July) and autumn (1st October), respectively. In summer, the maximum growth rate was 0.361 day-1 and others were from 0.04 to 0.34 day-1 with an average value of 0.12, four values were below zero; In autumn, the maximum growth rate was 0.111 day-1 and most values were below zero. Our results also demonstrated that the growth rate of Microcystis was higher inshore than offshore. © by PSP.