Concentrations of particulate phytochelatin, a metal‐binding peptide produced by eukaryotic phytoplankton, ranged from 2 to 50 µmol (g Chl a)−1 in samples collected from small harbors in southeastern New England. Although there was no obvious relationship between phytochelatin and total Cu concentrations, phytochelatin varied systematically with free Cu concentrations [Cu2+]. Transects in which there was high [Cu2+] revealed high phytochelatin concentrations with a general decrease seaward. In those where [Cu2+] remained low and constant, phytochelatin levels also remained low and constant. Incubation experiments confirmed that Cu rather than Cd is likely responsible for the elevated concentrations of phytochelatin at our field sites, though Zn may also be important. Intracellular phytochelatin concentrations in laboratory cultures of Skeletonema costatum, a coastal diatom, increased with increasing concentrations of Cu and Zn in a metal‐specific dose‐response relationship. Additions of 12 nM Zn' (inorganic Zn) to Cu‐stressed cultures reduced phytochelatin production, suggesting that Zn competitively inhibits uptake of Cu. Antagonistic effects of metals in the field, as well as physiological differences between organisms growing in the field and in the laboratory, can probably explain why phytochelatin concentrations in S. costatum are higher than in particulate field samples at the same [Cu2+].