Vital effects may be important in determining metal levels in bivalves and must be taken into account before the environmental ‘meaning’ of the data may be interpreted. Manganese concentrations in shells and soft tissues of several species of freshwater bivalves from three recent environments in northern Ohio and one archeological site from southern Ohio were determined by neutron activation analysis and show a species effect where Fusconaia flava Mn levels > Anodonta grandis grandis levels ≥ all Lampsilis species levels. Manganese is concentrated by a factor of 2 to 25 in soft tissues compared to shells. In addition, within-taxon variability of Mn concentration is high. The high intraspecies variability points to the necessity of processing many individuals before reliable numbers may be obtained.