ABSTRACTWe examined the effect of the presence or absence of coccoliths on the growth and sinking rates of an oceanic isolate of the coccolithophoreEmiliania huxleyi(Lohmann) Hay et Mohler isolated from the northeastern subarctic Pacific. Coccolith‐forming and non‐coccolith‐forming (i.e. naked, nonmotile) strains were obtained from the same isolate and grown under both saturating and limiting irradiance levels with either nitrate or ammonium as the primary nitrogen source. Sinking rate, growth rate, and cell volume (excluding coccoliths) were measured for each culture. Under saturating irradiance, coccolith‐forming cells grew significantly slower than naked cells, had significantly higher sinking rates, and had larger cell volumes than naked cells. Under limiting irradiance levels, growth rates of the two strains were identical, sinking rates were higher for coccolith‐forming cells in stationary‐phase cultures only, and cell volumes remained greater for coccolith‐forming cells.The sinking rates achieved for this ubiquitous coccolithophore ranged from<0.1 to 0.5 m · d−1. Sinking rates were not statistically different between coccolith‐forming and naked strains ofE. huxleyiunder limiting irradiance conditions for log‐phase cultures, but sinking rates were greater for coccolith‐forming cells under some of the other conditions tested. However, the average sinking rate was never more than twice as great as for coccolith‐forming cells, with the exception of nitrate‐grown, senescent cells under limiting irradiance (3.4 times greater). Cell volumes (excluding coccoliths) were consistently ca. 1.5 times greater for coccolith‐forming cells than for naked cells.Nitrogen source had an effect on growth rate and cell volume, with ammonium‐grown cultures growing faster and having larger cell volumes than nitrate‐grown cultures of both strains. However, despite the difference in growth rate and cell volume, nitrogen source had litt