(MFLS) were evaluated for the Poecilia reticulata in laboratory bioassays. The acute toxicity was evaluated for 96 h while the sub-lethal effects of the two surfactants on the biochemical parameters of P. reticulata were investigated for a 6 -day exposure period. The derived toxicity indices showed that the FLS (96hLC 50 = 0.062ml/L) was 1.37 times more toxic than the MFLS (96hLC 50 = 0.085ml/L) against P. reticulata. Analysis of variance (ANOVA) showed signifi cant difference (p<0.05) in the quantal response of P. reticulata to different concentrations of FLS and MFLS at 24, 48, 72 and 96 h of exposure. The mean tissue protein, triglyceride, glucose and cholesterol ranged from (21.04 -48.54 and 18.06 -58.54mg/dl), (87.43 -154.95 and 85.63 -144.28mg/dl), (33.95 -55.03 and 32.76 -53.15mg/dl) and (37.30 -65.38 and 37.12 -58.11mg/dl) for FLS and MFLS respectively. ANOVA showed signifi cant difference (p<0.05) in the biochemical response of P. reticulata to FLS and MFLS. The implications of these fi ndings are highlighted.