The division cycle of two phytoplankton species, Olisthodiscus luteus and Heterocapsa sp. was studied in relation to a 12:12 light:dark cycle. Batch cultures in exponential phase were sampled every three hours during 48 hours. Cell number, cellular volume and DNA and RNA concentrations were measured. Microscopic observations of the nuclei of Heterocapsa sp. were also performed. In both species, cell division took place in the dark. In Heterocapsa sp., DNA and RNA showed a similar diel variability pattern, with synthesis starting at the end of the light period, previously to mitosis and cytokinesis. In O. luteus. Major RNA synthesis occurred during darkness, and DNA was produced almost continuously. Both species presented different values and diel rhythmicity on the RNA/DNA ratios. This research was supported by FPI grants from the Ministerio de Educación y Ciencia de Espafla and funds from Programa Fronts MAR 88-0252, CYCIT, CSIC., and the Comissi6 Internacional per a la Recerca i la Innovació Tecnológica, CIRIT, de la Generalitat de Catalunya