Bacterial concentrations in natural sea water were significantly reduced by 5ptm filtration and subsequent treatment with ozone and UV light led to further reductions in total viable counts of bacteria and autoclaving water gave complete sterility. However, with the addition of artificial diets, such treated water was recolonised very rapidly and within 24h, these water samples had significantly higher bacteria counts than the 5pm filtered sea water treatment (f = 21.30; p<0.001). Further treatment of 5ýtrn filtered sea water also led to changes in the bacterial composition of the water. In ozonated and UV light irradiated water samples, the proportions of presumptive Vibrid sp. increased in the 24h following treatment while in 5ýLm filtered and natural sea water samples, the proportions of presumptive Vibrid sp. fell over the same period. When Penaeus indicus PZ1 stage larvae were reared to PI-1 stage in 5lam filtered sea water, good larval survival was obtained irrespective of whether live or microencapsulated artificial diets were fed. In contrast, larvae raised in autoclaved, ozonated and UV light irradiated water exhibited significantly lower survival when raised on live diets and often showed complete mortalities when fed on microencapsulated diets. In periods of poor sea water quality, additions of bacteria from algal cultures to PZ1 stage larvae fed algae had no effect on larval survival to M1 stage for larvae reared in 5gm filtered sea water or autoclaved water. In contrast, additions of these bacteria to larvae reared in 5pm filtered sea water and fed on MED, led to a significant increase in larval survival. This effect was absent when larvae were reared in autoclaved water. Addition of bacteria from algal cultures were found to inhibit counts of presumed viable Vibrid sp. while having no effects on total viable counts. No significant differences were observed in the percentage of soluble protein leached from microencapsulated diets and micro particulate diets when incubated up to 48h in bacterial laden and sterile water indicating that leaching is independent of microbial activity up to 48h. However broken microcapsules gave higher total viable bacterial counts over 48h in 5ýtm filtered sea water when compared with intact microcapsules. Significant levels of protection were conferred on larvae when either fresh or freeze-dried vaccines were administered by Immersion, but not when such vaccines were administered orally. The degree of protection offered was correlated with the virulence of the pathogen from which the vaccine was made. Enhanced protection given by vaccines produced from the more virulent strains was not wholly due to activation of the prophenoloxidase system since such vaccines induced less stimulation of the prophenoloxidase system than less virulent strains. Untreated plasma of Penaeus vannamei significantly enhanced Escherichia coli growth compared with sea water nutrient medium. In contrast, plasma from vaccinated prawns exhibited antibacterial activity detectable up to 7d after vaccination. Exposure to a mixture of fungicides Implicated in the initiation of the taura syndrome disease (TS), had no effects on the immune competence of P. indicus juveniles following live, in vivo pathogenic challenge. In addition, no effects on growth, growth rates, moulting rates or survival of postlarval and early juvenile prawns were observed following exposure to the fungicides. Prawns exhibited no gross or histopathological symptoms characteristic of TS.