Diel migration of Mesodinium rubrum, Eutreptiella sp., Scrippsiella tr
ochoidea, Dinophysis acuminata and Ceratium furca throughout a 24 h cy
cle is described for a stable, well-stratified estuary (Ria de Vigo, N
W Spain). Daily changes in light quantity and in spectral light ratios
i.e. red:far-red, blue:red, green:red and blue:green have been analys
ed. The spectral light ratios changed at twilight and around noon at v
arious depths. Some of the downward migrations were well predicted by
Stokes' law, while other migrations were faster and deeper than calcul
ated. The coincidence of these movements with abrupt changes in red:fa
r-red, green:red and blue:green light ratios is discussed. Some specie
s are able to migrate through the pycnocline, whereas others do not se
em to be able to do so. Several species are present in maximum numbers
at depth at night, while others display upward migration independent
of light, suggesting the existence of endogenous rhythms. Upward migra
tion at dusk began with dispersal of populations, with renewed aggrega
tion at the sea surface coincident with an increase in the red:far-red
ratio at 6 m and the green:red ratio at 6 and 10 m. Based on direct e
vidence for the control of flagellar mobility by light quality reporte
d by other authors from laboratory studies, it is suggested that, toge
ther with other cues, spectral light ratios of different light qualiti
es modulate vertical phytoplanktonic migration.