Soluble fractions of germinated and non-germinated Solanum tuberosum tubers were evaluated as nutrient sources for the microalga Phaeodactylum tricornutum in batch and semicontinuous unialgal cultures. In batch cultures, the soluble fraction of germinated potato produced the highest microalgal density, 84.7 x 10(6) cell ml(-1), compared with non-germinated potato, 66.4 x 10(6) cell ml(-1) and the autotrophic control, 54.8 x 10(6) cell ml(-1). The soluble fraction of germinated potato also increased cell and pigment productivities in semicontinuous cultures. Maximum cell productivities of 14.0 and 14.6 cell x 10(9) l(-1) d(-1) were obtained with renewal rates of 20 and 30% of the volume of the culture per day respectively. Maximum productivities of carotenoid (2.9 mg l(-1) d(-1)) and chlorophyll (7.5 mg(-1) d(-1)) were obtained with a renewal rate of 40%. When cultured axenically the non-germinated potato extract also produced a higher cell density of P. tricornutum than did the autotrophic control. The germinated potato constitutes an appropriate nutrient source for the production of microalgal biomass and pigments in mixotrophic conditions.