Bioassay‐measured, labile dissolved organic carbon (LDOC) concentrations were compared in near‐bottom and near‐surface Lake Michigan water between April and October 1986. In five of seven experiments, the LDOC concentration was higher in near‐bottom water. LDOC reached 40.2% of the total DOC pool in the near‐bottom water in late May and 13.8% in the near‐surface water in early July. Concentration in near‐bottom water was highest during early stratification; concentration in surface water varied less but was highest in early July. The data suggest that an allochthonous source of labile organic C may be important.