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The combined effect of pre®ltration and ultraviolet (UV) irradiation on the removal of bacteria associated with plankton fragments was investigated in an experimental¯owthrough system. The system was supplied by sea water containing fragments of Artemia nauplii in the size range 10±240 mm diameter (mean = 66 mm), as determined by particle size analysis. By applying an UV dose of 10 mJ cm ±2 , the number of colony-forming units (CFU) was reduced from 7.4 Q 10 5 per 100 mL to 2.0 Q 10 3 per 100 mL. Increasing the dose to 13 or 22 mJcm ±2 did not result in any further reduction, suggesting that bacteria associated with Artemia fragments were protected. This protection hypothesis was supported by improved overall bacterial removal ef®ciency by pre®ltration. At the highest dose (22 mJ cm ±2 ), more than 5 log 10 units total reduction was obtained by pre®ltration using 50mm mesh sizes. The reductions relative to the un®ltered sample were 1.4, 2.4 and 3.0 log 10 units using mesh sizes of 355, 80 and 50 mm respectively. It is concluded that sea water for aquacultural purposes should be ®ltered to remove particles before UV disinfection, thereby improving the overall bacterial removal ef®ciency and reducing the risk of introducing UV-shielded bacteria.