Co-culturing and using cell-free filtrates are common methods for investigating allelopathy of marine phytoplankton; however, these methods often yield inconsistent or even contradictory results. The induced release of allelopathic compounds has been hypothesized as a mechanism to explain the discrepancy. Here, we used experiments to assess the inducibility of allelopathy by the diatom, Thalassiosira weissflogii, on the colony formation of Phaeocystis globosa. T. weissflogii and its cell-free filtrates showed inhibitory effects on the growth of solitary P. globosa cells. The colony number, colony diameter, and cells per colony decreased by co-occurring T. weissflogii cells but were enhanced by their extracellular filtrates alone. Living T. weissflogii cells possibly affect the colony integrity by reducing colonial cell density of P. globosa. When P. globosa and T. weissflogii were co-cultured but separated with a 2-µm membrane filter, thus allowing the exchange of extracellular secretions without direct cell contact, P. globosa colony concentration, colony diameter, cells per colony and colonial cell density were inhibited. Once T. weissflogii cells were pre-exposed to cell-free filtrates of P. globosa, their filtrates inhibited colony formation. T. weissflogii had allelopathic effects on P. globosa by releasing extracellular compounds that inhibited growth of solitary cells and colony formation, as well as disrupting colony integrity. However, the allelopathic effects of T. weissflogii on colony formation were only induced when the presence of P. globosa was perceived. Chemically mediated allelopathic effects of diatoms on colony formation of P. globosa may play an important role in the succession of diatoms and Phaeocystis.