The distribution of tubificids in Kentucky Lake sediments is very patchy and even where worms are present, abundances are low.  Previous studies had showed potential correlations between worm abundance and sediment managanese and iron concentrations.  To assess the effects of manganese/iron, we resurveyed porewater along lake transects with high and low manganese/iron levels and conducted laboratory bioassays using whole sediments and sediments where manganese/iron levels were artificially elevated. The bioassay consisted of Tubifex tubifex sediment reworking rates that were measured in sediment microcosms using a gamma-scan biomonitoring system containing a ¹³⁷cesium marker layer.  Porewater analyses indicated that there were elevated manganese and iron levels on the eastern side of the lake (3.1 ppm to 14.2 ppm for manganese and 7.5 ppm to 43.2 ppm for iron). However, the porewater analysis across four transects did not support the east/west division in porewater concentrations. The gamma-scan experiment indicated that worms would not burrow into sediments from the eastern side of the lake. Worm burrowing and feeding activity rates decreased as manganese concentrations were artificially increased in bulk sediments. However, when iron was added to sediments at three levels (20.2, 60.0, and 100 mg/L) the worm activity rates were unaffected.  It appears that in Kentucky Lake, the distribution of tubificids is controlled by sediment managanese concentrations that most likely originate from tributary inputs.