Large-scale green tides of Ulva prolifera occur repeatedly in the Yellow Sea, and the microscopic propagules of U. prolifera play a critical role during the development of green tides. Ulva prolifera propagules and microalgae are both present in seawater and share similar niches, but their potential interactions are poorly understood. Nine species of microalgae were selected to study their interactions with the propagules of U. prolifera (gametes) in laboratory. The results showed that settlement of gametes could be inhibited by some microalgae, such as Alexandrium tamarense, Prorocentrum lima and Karenia mikimotoi, at the cell density of blooming (102–103 cells ml–1). Inversely, the germlings germinated from U. prolifera gametes had negative effects on the microalgae, the inhibition rate ranged from 28 to 66%. Our results demonstrated the complex interactions between microalgae and propagules of green algae, which may influence the formation of green tides and their ecological consequences in the Yellow Sea.