Electronic Journal of Differential Equations, Vol. 2010(2010), No. 98, pp. 1-15. Title: Stochastically perturbed allelopathic phytoplankton model Authors: Syed Abbas (Univ. degli Studi di Bologna, Italy) Malay Banerjee (Indian Inst. of Technology, Kanpur, India) Abstract: In this article we have considered a stochastic delay differential equation model for two species competitive phytoplankton system with allelopathic stimulation. We have extended the deterministic model system to a stochastic delay differential equation model system by incorporating multiplicative white noise terms in the growth equations for both species. We have studied the mean square stability of coexisting state using a suitable Lyapunov functional. Numerical simulation results are provided to validate the analytical findings. Submitted May 18, 2010. Published July 21, 2010. Math Subject Classifications: 93E15, 34C23, 37B25, 34K50. Key Words: Stability; bifurcation; Lyapunov functional; stochastic perturbation