Some species of marine phytoplankton are believed to be more tolerant of high concentrations of trace metals than others, but no conclusive test of this hypothesis has been conducted. Eleven species of phytoplankton representing 5 classes were grown in Aquil medium containing Cd$ sp{2+}$ concentrations between 10$ sp{-9.85}$ and 10$ sp{-6.84}$ M. Growth rates and intracellular concentrations of Cd, C, N and S were measured. Cadmium quotas (mol Cd/litre-cell volume) were lower in members of Bacillariophyceae than in Chlorophyceae, Prymnesiophyceae, Dinophyceae and Cyanophyceae (ANOVA, p $<$ 0.001). Cellular C:S molar ratios decreased in phytoplankton grown at high (pCd 7.37-6.84) compared to low Cd (no added Cd), as S/litre-cell volume increased. Similar results were observed for C:N molar ratios. In two species that were examined, C:S ratios decreased as a linear function of increasing Cd concentration. Mean Cd$ sp{2+}$ concentration that reduced growth rate to 50% of maximum (pCd$ sp{50})$ was not significantly different among phytoplankton classes (ANOVA, p $<$ 0.05). When these experimental data were combined with pCd$ sp{50}$s calculated from published sources, Chlorophyceae were found to be the most resistant class (ANOVA, p $<$ 0.01). Cadmium and Cu resistance (pCd$ sp{50}$ and pCu$ sp{50})$ were correlated (r = 0.52, p $<$ 0.05), suggesting co-tolerance of phytoplankton to toxic levels of these metals. Chlorophyceae were most tolerant and Cyanophyceae the least tolerant of Cu (ANOVA, p $<$ 0.01). No significant differences were observed among Bacillariophyceae, Prymnesiophyceae, and Dinophyceae, which were of intermediate sensitivity to both metals. The results confirm the existence of a phylogenetic dependence of resistance to trace metal toxicity in phytoplankton.

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