To investigate the effects of chlorella on methylmercury (MeHg) transfer to the fetus during pregnancy, female C57BL/6N mice (aged 10 weeks) were housed for 7 to 8 weeks, from 4 weeks before mating to birth, with diets containing 0% or 10% chlorella powder (CP) and MeHg-containing drinking water (2 µg Hg/ml). The consumption volume of the MeHg-containing water was limited to 15 ml/mouse/week throughout the experiment. Distilled water and a basal diet (0% CP) was given to control mice. Except for the mating period, during the 5(th) week, mice were housed individually until parturition. Two neonates were randomly selected from each mother mouse within 24 hr after parturition for Hg analysis of the blood, brain, liver, and kidneys. Mother mice were sacrificed on the same day as neonates to obtain tissue samples for Hg analysis. The blood and brain Hg levels of both neonates and mothers in the CP diet group were significantly lower than those in the basal diet group. Although the hepatic and renal Hg levels were not significant in mothers between the two dietary groups, in neonates, the CP diet group showed significantly lower Hg levels in these tissues than the basal diet group. The results obtained here revealed that continuous CP intake suppressed MeHg transfer to the fetus, in addition to effective suppressing MeHg accumulation in brains of the mothers.