Abstract: Aphanizomenon and Microcystis are common dominant genera of bloom-forming cyanobacteria, and they have seasonal succession in some lakes. Aphanizomenon gracile is the most common Aphanizomenon species in Chinese freshwater bodies which can produce odor substance geosmin. The effects of the interspecific interaction between Microcystis and Aphanizomenon on the cell growth and the synthesis and release of geosmin are not clear. In this paper, two Microcystis species with different characteristics, toxic Microcystis aeruginosa and non-toxic Microcystis wesenbergii, were respectively co-cultured with geosmin producing Aphanizomenon gracile at different initial inoculation ratios (1﹕2, 1﹕1 and 2﹕1) to explore the effects of interspecific interaction on algae growth and geosmin synthesis and release. The results showed that both Microcystis species inhibited the growth of Aphanizomenon, but the latter promoted the growth of the formers. Microcystis wesenbergii promoted the release of geosmin (when the initial inoculation ratio was 1﹕1, the extracellular geosmin reached 269.43 fg/cell), and promoted the synthesis of geosmin only in the early and late growth stages; Microcystis aeruginosa promoted the synthesis of geosmin at the early stage of co-culture, but co-culture inhibited the release of geosmin, and geosmin was not detected in the middle and late stages of co-culture. Our research results showed that during the seasonal succession of Aphanizomenon and Microcystis in natural water bodies, Microcystis has an advantage in the competition with Aphanizomenon, and the competitive pressure of Microcystis on Aphanizomenon urges it to synthesize odor substances. As the Aphanizomenon decays, it may be accompanied by the release of a large amount of odor substances, which increases the risk of odor events.