In the present study effect of the exogenously applied phytohormone, Indole, 3-acetic acid, on two marine diatoms viz. Skeletonema costatum and Thalassiosira fluviatilis was investigated. The phytohormonal impact on these two diatoms was measured in terms of growth rate and morphological characteristics over a period of 12 days. The effect of lAA was both promotory and inhibitory to growth and cell elongation, the extent being dependent on the concentrations. Maximum growth promoting concentrations were 0.1 and 1.0 ppm for S. costatum and T. fluviatilis respectively, while the highest concentration (5.0 ppm) used in the present investigation inhibited the growth and cell division of both the test organisms. S. costatum was comparatively more sensitive to higher concentrations of lAA than T. fluviatilis, indicating their differential auxin requirement for growth.