Process length variation of cysts of the dinoflagellate Protoceratium reticulatum (Claparede et Lachmann) Butschli in surface sediments from the North Pacific was investigated. The average process length showeda significant inverse relation to annual seawater density: _{t annual}= -0.8674 x average process length +1029.3(R²=0.84), with a standard error of 0.78 kgm^-3. A sediment trap study from Effingham Inlet in British Columbiarevealed the same relationship between average process length and local seawater density variations. In the Baltic–Skagerrak region, the average process length variation was related significantly to annual seawater density: _{t annual} =3.5457 x average process length -993.28 (R²=0.86), with a standard error of 3.09 kgm³. These calibrations cannot be reconciled, which accentuates the regional character of the calibrations. This can be related to variations in molecular data (small subunit, long subunit and internal transcribed spacer sequences), whichshow the presence of several genotypes and the occurrence of pseudo-cryptic speciation within this species.