The use of biomarkers in assessing the health of aquatic ecosystems is widely being used. Biomarkers have been proposed as sensitive early warning tools for biological measurement in environmental quality assessment. In this study, DNA damage, HSP 70 expression, Lactate dehydrogenase and alkaline phosphatase were evaluated for their usefulness and applicability in the active biomonitoring of the Rietvlei system using Melanoides tuberculata and Oreochromis mossambicuss as test organisms. Levels of DNA strand breakage were measured in the exposed test organisms and the references that were kept in the laboratory under unpolluted environment. Different levels of DNA damage were recorded in M tuberculata that were exposed during high-flow and those from the low-flow exposure period. There was no difference in DNA between the control and high-flow exposure M. tuberculata but the low-flow exposure duration had significantly higher DNA damage for all the three sites when compared to the high-flow and the controls. There was no difference in the amount of DNA damage in both exposed and control O. mossambicus. Fish samples were only available for the high-flow four weeks exposure because of the test organisms dying during the low-flow exposure. Heat shock protein 70 expression was determined in O. mossambious only and significant differences in expression between sites were demonstrated with highest expression at site 1 decreasing down stream at sites 3 and 5. The activity of LDH in M. tuberculata was not different in both control and exposed fish at all the sites. There was no difference in the activity of LDH between the high-flow and low-flow exposure groups in M. tuberculata. In O. mossambicus, the activity of LDH was significantly inhibited at site 1 with the effects of the contaminants decreasing downstream at sites 3 and 5 where LDH activity did not differ from the controls. There was no alkaline phosphatase activity in both exposed and control M. tuberculata but in O. mossambicus alkaline phosphatase was significantly inhibited at the three sites although inhibition did not differ from site to site. This suggests that LDH and alkaline phosphate are not sensitive biomarkers in M. tuberculata in the Rietvlei system but are sensitive in O. mossambicus. DNA damage was shown to be a sensitive biomarker using M. tuberculata but it was difficult to assess this in O. mossambicus since fish were only available during the 4 weeks high-flow exposure, which did not show any differences from the control. The lack of DNA damage in O. mossambicus maybe a result of low concentration of contaminants during high-flow or because this biomarker is not sensitive in fish. / Prof. J.H.J. Van Vuren