A study was undertaken to compare the tolerance lesel o f Sepat Siam and Tilapia to malathion. Fish reaction and behaviour when exposed to malathion was noted and discussed. The 96-hr LCS0 values fo r Tilapia and Sepat Siam were 0.29 ppm and 0.98 ppm respectively. Reasons fo r high tolerance exhibited by Sepat Siam are given and discussed.