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Tilapia and charal are important in the fishery of Lake Chapala, Mexico and are known to carry large body burdens of heavy metals. Copper and zinc concentrations in tilapia tissues were greatly reduced when the fish were exposed for one month to the metals in mesocosms containing suspended clay as compared with clay-free mesocosms. For charal, there was no apparent effect of clay on tissue accumulation of metal. Suspended clay is a feature of lakes and reservoirs-particularly in arid and semi-arid regions and in estuaries. Anthropogenic increases in clay suspensoids from dredging, land clearing, road construction and resuspension with declining water depths is occurring. These data suggest that such suspensoids, while having other ecosystem effects, provide a level of protection to biota from xenobiotic materials.