We investigated the link between iron (Fe) limitation and intracellular dimethylsulfoniopropionate (DMSP) concentration in two oceanic phytoplankton species, the diatom Thalassiosira oceanica and the diazotrophic cyanobacterium Trichodesmium erythraeum. Dimethylsulfoxide (DMSO) concentrations were also measured in Fe-replete and Fe-limited T. oceanica. Fe limitation decreased the growth rates of T. oceanica and T. erythraeum by 33-fold and 3.5-fold, respectively and increased intracellular DMSP (DMSPp) concentrations by 12-fold (from 2.8 to 33.7 mmol Lcell−1) and by 45-fold (from 0.05 to 2.27 mmol Ltrichome−1), respectively. Intracellular dimethylsulfoxide (DMSOp) concentrations in T. oceanica increased by 5-fold under severe Fe limitation, from 0.78 mmol Lcell−1 in Fe-replete cells to 3.86 mmol Lcell−1. The increase in DMSPp and DMSOp under Fe limitation provides support for the role of these sulfur compounds as antioxidants. Under severe Fe limitation, the large increase in DMSPp : C and DMSP : chlorophyll a (Chl a) ratios for both T. oceanica (by 16- and 40-fold, respectively) and T. erythraeum (by 18- and 145-fold, respectively) places these species above the range of values generally attributed to diatoms and cyanophytes. Comparison of these values with in situ results, such as those from Fe fertilization experiments, suggests that the decrease in DMSPp : Chl a and DMSOp : Chl a that is generally observed with alleviation of Fe limitation may be partly related to decreases in DMSPp and DMSOp in individual species. The role of diatoms and diazotrophic cyanobacteria in the biogeochemical cycle of dimethylsulfide and associated sulfur compounds in Fe-limited oceanic environments should not be overlooked.