The photosynthetic productivities of the marine microalgae Chlorophyta sp. and freshwater microalgae Chlorella sp. were investigated in a raceway system under greenhouse conditions in Sendai city. The system was constructed with a surface area of 0.986 m(2) and equipped with paddle wheels (8 fins). The semicontinuous batch culture experiment was carried out for 4 months from June to October 1998 in Sendai city. The productivity of Chlorophyta sp. was ranged from 4 to 13 g(.)m(-2.)d(-1) and the average was 8.2 g(.)m(-2.)d(-1), which corresponded to the photosynthetic efficiency of 4.15% (PAR). The productivity of Chlorella sp. was ranged from 7 to 21 g(.)m(-2.)d(-1) and the average was 13.2 g(.)m(-2.)d(-1) which corresponded to the photosynthetic efficiency of 6.56% (PAR). These results indicate that stable microalgal cultivation with comparatively high photosynthetic efficiency could be obtained in a raceway system under greenhouse conditions in Sendai city located in the northern part of Japan.