Abstract
The acclimated reproductive rates of 73 clones of Emiliania huxleyi and 19 clones of Gephyrocapsa oceanica were measured. Statistically significant genetic variability in reproductive rates was found among clones from single water bottles in both species. Genetic differentiation was found between populations from different water masses, but not within water masses. The spatial patterns of genetic differentiation appear to be different in the two species.