We investigated the effect of Zn availability on growth rate (μ), cell morphology, and elemental stoichiometry and incorporation rate in two marine diatoms. For the coastal diatom Skeletonema costatum (Grev.) Cleve, the half-saturation constant (KS) for growth was 4.1 pM Zn2+, and growth ceased at ≤ 2.6 pM Zn2+, whereas for the oceanic diatom Thalassiosira oceanica Hasle, KS was 0.5 pM Zn2+, and μ remained at ∼40%μmax even at 0.3 pM Zn2+. Under Zn-limiting (Zn-L) conditions, S. costatum decreased cell size significantly, leading to an 80% increase in surface area to volume ratio (SA/V) at Zn2+ of 3.5 pM compared to Zn-replete (Zn-R) conditions (at Zn2+ of 13.2 pM), whereas T. oceanica’s morphology did not change appreciably. Cell quotas of C, N, P, Si, and chl a significantly decreased under Zn limitation in S. costatum (at Zn2+ of 3.5 pM), whereas Zn limitation in T. oceanica (at Zn2+ of 0.3 pM) had little effect on quotas. Elemental stoichiometry was ∼85C:10N:9Si:1P and 81C:9N:5Si:1P for S. costatum, and 66C:5N:2Si:1P and 52C:6N:2Si:1P for T. oceanica, under Zn-R and Zn-L conditions, respectively. Incorporation rates of all elements were significantly reduced under Zn limitation for both diatoms, but particularly for Si in S. costatum, and for C in T. oceanica, despite its apparent tolerance of low Zn conditions. With [Zn2+] in some parts of the ocean being of the same order (∼0.2 to 2 pM) as our low Zn conditions for T. oceanica, our results support the hypothesis that in situ growth and C acquisition may be limited by Zn in some oceanic species.