The micronucleus test (MNT) in fish erythrocytes has increasingly been used to detect the genotoxic effects of environmental mutagens and its frequency is considered to reflect the genotoxic damage to cells, mainly the chromosomes. Besides, morphologically altered erythrocyte is taken as an index of cytotoxicity. Both parameters were used in the present study by two herbicides, 2,4-dichlorophenoxyacetic acid (2,4-D, in 25, 50 and 75ppm concentrations) and 2-chloro-2,6-diethyl-N-(butoxymethyl) acetanilide (butachlor, in 1, 2 and 2.5ppm concentrations) for genotoxic and cytotoxic endpoints. The study was carried out by an in vivo method on peripheral erythrocytes of catfish Clarias batrachus using multiple sampling times (48, 72 and 96h). Cytogenetic preparations were made by haematoxylin-eosin staining technique. Pycnotic and granular micronuclei (MN) were consistently observed irrespective of chemical tested. A wide range of altered cells was also observed. Echinocytes accompanied by altered nuclei and vacuoles were prominent feature of 2,4-D, whereas, anisochromasia and anisocytosis of erythrocytes were characteristic of butachlor. Increase in MN as well as altered cells frequencies were significant. A positive dose-response relationship in all exposures and sampling times was observed. Herbicides used were found to be genotoxic as well as cytotoxic in this fish. The suitability of the adopted parameters for the screening of the aquatic genotoxicants is discussed.