The Mo contents and the relations between Mo and Cu or Cr contents were investigated in the organs of Japanese wild ducks (spotbill duck, pintail, wigeon, scaup and tufted duck). The highest Mo content in kidney and liver of the dabbling ducks were more than 30 mug g(-1) dry weight (mug g(-1) d. wt.), though that of diving ducks were less than 11 mug g(-1) d. wt. The contents were lower in the ducks migrating within Japan, Eurasia and North America than those in the birds migrating between Japan and Eurasia. The contents of liver in all species were more than 50 and less than 5 mug g(-1) d. wt. for Cu and Cr, respectively. Significant correlations were found between Mo and both elements in pintail and scaup, and Mo and Cr in tufted duck. These results suggest that the contamination of wild ducks reflects the reproductive area, and not the collected area. Mo contents closely correlated with the Cu and/or Cr contaminations.