In this study, we evaluated the toxic effects of heavy metals (As, Cr and Pb) on the population growth rate (r) of the marine diatom, Skeletonema costatum. The r of the species was determined after 96 h (4 days) exposure to As (0, 0.31, 0.63, 1.25, 2.50 and 5.00 mg L-1), Cr (0, 0.16, 0.31, 0.63, 1.25 and 2.50 mg L-1) and Pb (0, 1.25, 2.50, 5.00, 10.00 and 20.00 mg L-1). It was observed that r in the control (absence of As, Cr and Pb) were greater than 0.04, however suddenly decreased with increased heavy metal concentrations. As, Cr and Pb reduced r in a dose-dependent manner and a significant reduction were occurred at concentration of greater than 0.31, 0.31 and 5.00 mg L-1, respectively. Based on the toxicity, the heavy metal were ranked as Cr>As>Pb, with EC50 (50% effective concentration) values of 0.80, 2.57 and 13.88 mg L-1, respectively. The lowest-observed-effective-concentration (LOEC) of r in exposed to As, Cr and Pb were 0.63, 0.63 and 10.00 mg L-1, respectively. Precisely, a concentration of greater than 0.63 mg L-1 of As, 0.63 mg L-1 of Cr and 10.00 mg L-1 of Pb in marine ecosystems induced toxic effect on the r of S. costatum. Based on our results, we suggested that the r of S. costatum might be a useful bio-indicator for the toxicity assessment of heavy metals on the producer in marine ecosystems.