Abstract: Water hyacinth infested and open water areas in AWBA reservoir were sampled fortnightly between July and November 2005. The effect of water hyacinth infestation on the aquatic biota was assessed using physico-chemical parameters. The study showed that conductivity, total suspended solids, free carbon dioxide and turbidity were significantly higher (p<0.05) in water hyacinth infested area than in open water. Dissolved oxygen, pH and nitrate-nitrogen were significantly lower (p<0.05) in the infested areas when compared with open water. Temperature was higher in the infested area than open water but was also not significant (p>0.05). The study suggested that water hyacinth infestation negatively impacted on the reservoir and therefore should be eradicated.